Readout circuits and methods

ABSTRACT

Methods of sensor readout and calibration and circuits for performing the methods are disclosed. In some embodiments, the methods include driving an active sensor at a voltage. In some embodiments, the methods include use of a calibration sensor, and the circuits include the calibration sensor. In some embodiments, the methods include use of a calibration current source and circuits include the calibration current source. In some embodiments, a sensor circuit includes a Sigma-Delta ADC. In some embodiments, a column of sensors is readout using first and second readout circuits during a same row time.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application No.62/659,048, filed on Apr. 17, 2018, U.S. Provisional Application No.62/757,078, filed on Nov. 7, 2018, and U.S. Provisional Application No.62/819,376, filed on Mar. 15, 2019, the entire disclosures of which areherein incorporated by reference for all purposes.

FIELD OF THE INVENTION

This disclosure generally relates to MEMS sensors. More specifically,this disclosure relates to sensor readout circuits, calibrationcircuits, and methods corresponding to the circuits.

BACKGROUND OF THE INVENTION

A MEMS sensor array can convert a sensor image into a plurality ofindividual pixel signals. In a bolometer, for example, an array ofbolometer pixels are exposed to a thermal image (e.g., a spectrum ofwaves in the Long Wavelength Infrared range, hereinafter “LWIR”). Inresponse to the exposure, an impedance changes between two terminals ineach bolometer pixel and the changes are then captured as signalsrepresentative of the thermal image.

SUMMARY OF THE INVENTION

MEMS sensors must contend with interference. In the bolometer example,interference can be overwhelming. Bolometer interference can manifest aslarge ambient interferences common to all the pixels in the array thatarises from changes in the ambient condition and from self-heating ofresistive elements (e.g., resistors in the circuit, the bolometer pixelsthemselves). These common mode changes are typically large (e.g., up to+/− 50 K) compared to a minimum detectable thermal image signal on agiven pixel (e.g., about 0.5 mK). In other words, bolometer noisedominates the intended signal by many orders of magnitude.

Sensor noise can affect the clarity of a sensor image. Noise in abolometer array can include both temporal and spatially-patterned noise.Examples of noise include 1/f noise, thermal noise, and processdependent variations. When the signals in the array are read out bycircuits arranged according to columns and rows, variations in the readout can lead to visible row-to-row and column-to-column stripes.

Under traditional configurations, common mode changes are undesirablyoutputted for measurement, reducing the dynamic range of subsequentmeasurement or sampling stages. Some known solutions add circuitry toreduce undesired common mode effects at measurement, but these solutionshave a small environmental range of applicability and poorrepeatability, require manual trimming or calibration, increasecomplexity, size, and cost, introduce more parasitics and unknowns, andconsume more power, to name but a few disadvantages.

Examples of the disclosure are directed toward MEMS sensor readoutcircuits and methods that overcome the herein-identified drawbacks. Insome embodiments, sensor readout circuits include a reference sensor, anactive sensor, current sources, a voltage driver, and a readout element.In some embodiments, methods include providing a current to a referencesensor, generating a common mode-tracking bias voltage electricallycoupled to an active sensor, and measuring a current change at a readoutelement.

As an exemplary advantage, the disclosed circuits and methods reducecommon mode effects without the cost and complexity of additionalcircuitry. The circuits presented herein can efficiently and compactlytrack common mode changes in the sensor array. Thus, accuracy of themeasured thermal image signal can be improved, and the input rangerequirement of subsequent analog-to-digital converters (ADC) can bereduced without additional components for compensation. By trackingcommon mode changes with a bias voltage, speed, and accuracy can beimproved because column voltages can be nominally at fixed voltages andunaffected by column parasitic elements.

In some embodiments, a sensor readout circuit includes a readoutelement, a first current source, a second current source, a voltagedriver, a reference sensor, and an active sensor. The readout elementincludes an input. The voltage driver includes an output. The referencesensor includes a first terminal and a second terminal; the firstterminal is electrically coupled to the first current source and thesecond terminal is electrically coupled to the output of the voltagedriver. The active sensor includes a first terminal and a secondterminal; the first terminal is electrically coupled to the secondcurrent source and the input of the readout element and the secondterminal is electrically coupled to the output of the voltage driver.The active sensor is configured for exposure to a sensor image.

In some embodiments, the first current source and the second currentsource are constant sources.

In some embodiments, the voltage driver generates a bias voltage for theactive sensor.

In some embodiments, the active sensor is further configured to change acurrent from the first terminal of the active sensor to the input of thereadout element when the active sensor is exposed to the sensor image.

In some embodiments, the active sensor is further configured to changean impedance of the active sensor when the active sensor is exposed tothe sensor image.

In some embodiments, the reference sensor is a reference bolometer pixeland the active sensor is an active bolometer pixel.

In some embodiments, the circuit further includes a second referencesensor, a second active sensor, a first switch, a second switch, a thirdswitch, and a fourth switch. The second reference sensor includes afirst terminal and a second terminal; the first terminal is electricallycoupled to the first current source and the second terminal iselectrically coupled to the voltage driver. The second active sensorincludes a first terminal and a second terminal; the first terminal iselectrically coupled to the second current source outputting the secondcurrent and the second terminal is electrically coupled to the output ofthe voltage driver. The second active sensor is configured to change thecurrent from the first terminal to the input of the readout element. Thefirst switch is configured to selectively electrically couple thereference sensor to the first current source. The second switch isconfigured to selectively electrically couple the active sensor to thesecond source. The third switch is configured to selectivelyelectrically couple the second reference sensor to the first currentsource. The fourth switch is configured to selectively electricallycouple the second active sensor to the second current source.

In some embodiments, the circuit further includes a correlated doublesampling (CDS) circuit that is configured to remove an offset.

In some embodiments, the voltage of the readout element is proportionalto an impedance difference between the reference sensor and the activesensor.

In some embodiments, the circuit further includes an output of an op ampelectrically coupled to the second terminal of the reference sensor.

In some embodiments, the circuit further includes a feedback elementthat is electrically coupled to the first and second terminals of thereference sensor.

In some embodiments, the circuit further includes a third referencesensor and a third current source. The third reference sensor includes afirst terminal and a second terminal that is electrically coupled to theoutput of the voltage driver. The third current source is electricallycoupled to the first terminal of the third reference sensor, and isconfigured to output a seventh current reflective of self-heatinggenerated by the third reference sensor. The value of the second currentadjusts in accordance with the seventh current.

In some embodiments, the circuit further includes an ADC that isconfigured to sample the change of the current from the first terminalto the input of the readout element.

In some embodiments, the first current source and the second currentsource are configured to output an equal magnitude of current in a samedirection relative to the respective first terminals.

In some embodiments, the readout element includes a capacitivetransimpedance amplifier (CTIA).

In some embodiments, the first current source and the second currentsource are selected from the group of an athermal voltage source andresistor, a high-impedance athermal transistor current source, and aWilson current mirror.

In some embodiments, non-sensor elements of the readout circuit aredesigned to be substantively athermal and/or minimize the effects ofself-heating.

In some embodiments, the circuit further includes an amplifier thatoutputs to the second terminal of the reference sensor. The firstterminal of the reference sensor electrically couples to a negativeinput of the amplifier. The first current source is configured togenerate a voltage drop across the negative input and the output.

In some embodiments, the reference sensor is a reference bolometerpixel, and the active sensor is a bolometer pixel configured to detectLWIR radiation.

In some embodiments, the readout element includes a Sigma-Delta ADC.

In some embodiments, a first stage of the Sigma-Delta ADC includes aCTIA.

In some embodiments, the reference sensor is shielded from a sensorimage.

In some embodiments, the circuit further includes a voltage followerelectrically coupled between the output of the voltage driver and thesecond terminal of the active sensor.

In some embodiments, the circuit further includes two or more currentbuffers, the two or more current buffers including a first currentbuffer electrically coupled between the first current source and thereference sensor and a second current buffer electrically coupledbetween the second current source and the active sensor.

In some embodiments, the circuit further includes a fifth switchconfigured to selectively electrically couple the active sensor to thevoltage driver.

In some embodiments, a method of sensor readout includes: providing afirst current to a first terminal of a reference sensor; generating,from the first current, a voltage at a second terminal of the referencesensor; providing a second current to a first terminal of an activesensor; driving, at the voltage, a second terminal of the active sensor;exposing the active sensor to a sensor image; and measuring a thirdcurrent from the first terminal of the active sensor to an input of areadout element.

In some embodiments, the first current and the second current areconstant.

In some embodiments, the voltage is a bias voltage for the activesensor.

In some embodiments, exposing the active sensor to the sensor imagefurther includes changing the third current.

In some embodiments, exposing the active sensor to the sensor imagefurther includes changing an impedance of the active sensor.

In some embodiments, the method further includes: providing a fourthcurrent to a first terminal of a second reference sensor; generating,from the fourth current, a second voltage at a second terminal of thesecond reference sensor; providing a fifth current to a first terminalof a second active sensor; driving, at the second voltage, a secondterminal of the second active sensor; exposing the second active sensorto the sensor image; and measuring a sixth current from the firstterminal of the second active sensor to the input of a readout element.

In some embodiments, the method further includes: electricallyuncoupling, from the reference sensor, a first current source providingthe first current; coupling, to the second reference sensor, the firstcurrent source providing the fourth current; electrically uncoupling,from the active sensor, a second current source providing the secondcurrent; and coupling, to the second active sensor, the second currentsource providing the fifth current.

In some embodiments, the method further includes: determining an offsetgenerated by the input of the readout element; and canceling the offsetprior to measuring the current to the input of the readout element.

In some embodiments, a voltage at an output of the readout element isproportional to an impedance difference between the reference sensor andthe active sensor.

In some embodiments, the voltage is driven by an op amp, and the firstterminal of the reference sensor is electrically coupled to a negativeinput of the op amp.

In some embodiments, the method further includes feeding back from thesecond terminal of the reference sensor to the first terminal of thereference sensor.

In some embodiments, the method further includes: providing a seventhcurrent to a first terminal of a third reference sensor, the seventhcurrent reflective of self-heating generated by the third referencesensor; and adjusting a value of the second current in accordance withthe seventh current.

In some embodiments, the method further includes sampling a voltagegenerated by the current to the input of a readout element.

In some embodiments, the first current and the second current are equalin magnitude and in a same direction relative to the respective firstterminals of the reference sensor and active sensor.

In some embodiments, the method further includes converting the thirdcurrent to a readout voltage of the readout element.

In some embodiments, providing the first current and providing thesecond current each includes providing at least one selected from thegroup of an athermal voltage source and resistor, a high-impedanceathermal transistor current source, and a Wilson current mirror.

In some embodiments, driving the second terminal of the active sensor atthe voltage further includes driving, from an output of a voltagedriver, the second terminal of the reference sensor and the secondterminal of the active sensor.

In some embodiments, the method further includes causing a voltage dropacross the reference sensor from the first current; generating thevoltage using an amplifier outputting to the second terminal of thereference sensor; and electrically coupling the first terminal of thereference sensor to a negative terminal of the amplifier.

In some embodiments, the reference sensor is a reference bolometer pixeland the active sensor is an active bolometer pixel.

In some embodiments, exposing the active sensor to the sensor imagefurther includes exposing the active sensor to LWIR radiation.

In some embodiments, the readout element includes a Sigma-Delta ADC.

In some embodiments, a first stage of the Sigma-Delta ADC includes aCTIA.

In some embodiments, the method further includes exposing the referencesensor to an ambient condition common to the reference sensor and theactive sensor; and shielding the reference sensor from the sensor image.

In some embodiments, driving the second terminal of the active sensor atthe voltage further includes buffering between the second terminal ofthe active sensor and a voltage source providing the voltage.

In some embodiments, the method further includes: buffering the firstcurrent; and buffering the second current.

In some embodiments, a method of manufacturing a sensor readout circuitincludes providing a readout element including an input; providing afirst current source; providing a second current source; providing avoltage driver including an output; providing a reference sensorincluding a first terminal and a second terminal; electrically couplingthe first terminal of the reference sensor to the first current source;electrically coupling the second terminal of the reference sensor to theoutput of the voltage driver; providing an active sensor including afirst terminal and a second terminal, the active sensor configured forexposure to a sensor image; electrically coupling the first terminal ofthe active sensor to the second current source and the input of thereadout element; and electrically coupling the second terminal of theactive sensor to the output of the voltage driver.

In some embodiments, the first current and the second current sourcesare constant current sources.

In some embodiments, the voltage driver is configured to generate a biasvoltage for the active sensor.

In some embodiments, the active sensor is further configured to change acurrent from the first terminal of the active sensor to the input of thereadout element when the active sensor is exposed to the sensor image.

In some embodiments, the active sensor is further configured to changean impedance of the active sensor when the active sensor is exposed tothe sensor image.

In some embodiments, the reference sensor is a reference bolometer pixeland the active sensor is an active bolometer pixel.

In some embodiments, the method of manufacturing further includes:providing a second reference sensor including a first terminal and asecond terminal; electrically coupling the first terminal of the secondreference sensor to the first current source; electrically coupling thesecond terminal of the second reference sensor to the voltage driver;providing a second active sensor including a first terminal and a secondterminal, the second active sensor configured for exposure to the sensorimage; electrically coupling the first terminal of the active sensor tothe second current source; electrically coupling the second terminal ofthe active sensor to the output of the voltage driver, and the secondactive sensor is configured to change a current from the first terminalof the active sensor to the input of the readout element; and providinga first switch configured to selectively electrically couple thereference sensor to the first current source; providing a second switchconfigured to selectively electrically couple the active sensor to thesecond current source; providing a third switch configured toselectively electrically couple the second reference sensor to the firstcurrent source; and providing a fourth switch configured to selectivelyelectrically couple the second active sensor to the second currentsource.

In some embodiments, the method of manufacturing further includesproviding a CDS circuit configured to remove an offset.

In some embodiments, the readout element is configured to generate avoltage proportional to an impedance difference between the referencesensor and the active sensor.

In some embodiments, the method of manufacturing further includes:providing an op amp; and electrically coupling an output of an op amp tothe second terminal of the reference sensor.

In some embodiments, the method of manufacturing further includes:providing a feedback element; and electrically coupling the feedbackelement to the first and second terminals of the reference sensor.

In some embodiments, the method of manufacturing further includes:providing a third reference sensor including a first terminal and asecond terminal; electrically coupling the second terminal of the thirdreference sensor to the output of the voltage driver; providing a thirdcurrent source configured to output a seventh current reflective ofself-heating generated by the third reference sensor, and a value of thesecond current adjusts in accordance with the seventh current; andelectrically coupling the third current source to the first terminal ofthe third reference sensor.

In some embodiments, the method of manufacturing further includesproviding an ADC configured to sample the change of the current from thefirst terminal to the input of the readout element.

In some embodiments, the first current source and the second currentsource are configured to output an equal magnitude of current in a samedirection relative to the respective first terminals.

In some embodiments, the readout element includes a CTIA.

In some embodiments, the first current source and the second currentsource are selected from the group of an athermal voltage source andresistor, a high-impedance athermal transistor current source, and aWilson current mirror.

In some embodiments, the method of manufacturing further includes:providing an amplifier outputting to the second terminal of thereference sensor; and electrically coupling the first terminal of thereference sensor to a negative input of the amplifier, the first currentsource configured to generate a voltage drop across the negative inputand the output.

In some embodiments, the reference sensor is a reference bolometer pixeland the active sensor is a bolometer pixel configured to detect LWIRradiation.

In some embodiments, the readout element includes a Sigma-Delta ADC.

In some embodiments, a first stage of the Sigma-Delta ADC includes aCTIA.

In some embodiments, the reference sensor is shielded from a sensorimage.

In some embodiments, the method of manufacturing further includes:providing a voltage follower; and electrically coupling the voltagefollower between the output of the voltage driver and the secondterminal of the active sensor.

In some embodiments, the method of manufacturing further includes:providing two or more current buffers including a first current bufferand a second current buffer; electrically coupling the first currentbuffer between the first current source and the reference sensor; andelectrically coupling the second current buffer between the secondcurrent source and the active sensor.

In some embodiments, the methods include measuring a voltage of thecalibration sensor and computing a calibrated readout voltage based onthe measured calibration sensor voltage. In some embodiments, themethods include measuring a readout voltage of a readout elementelectrically coupled to the calibration current source and computing anoutput based on the readout voltage caused by the calibration current.In some embodiments, the methods include measuring readout voltages whenthe shutter is closed and when the shutter is opened and computing adifference between the readout voltages.

In some instances, row-to-row pattern noises can be caused by noise inthe bias voltage, which can be common for a row of sensors. Therefore,the noise of the bias voltage is observed for an entire row of sensors.In some instances, column-to-column pattern noises can be caused bymismatch and 1/f noise of skimming currents and ADC associated withspecific columns of sensors.

Examples of the disclosure are directed toward MEMS sensor calibrationcircuits and methods that overcome the herein-identified drawbacks(e.g., pattern noises). In some embodiments, the sensor calibrationcircuits include a calibration sensor and a calibration readout element.In some embodiments, the sensor calibration circuits include acalibration current source. In some embodiments, a shutter is includedwith the sensor calibration circuits.

As an exemplary advantage, the disclosed circuits and methods removenoise at reduced costs. The circuits presented herein efficiently andcompactly remove noises in the sensor array. Thus, the clarity ofmeasured sensor images can be improved.

In some embodiments, a sensor circuit includes: a plurality of activesensors exposed to a sensor image and sharing a bias voltage node; acalibration readout element; and a calibration sensor shielded from thesensor image and including a first terminal electrically coupled to thebias voltage node and a second terminal electrically coupled to thecalibration readout element.

In some embodiments, an impedance of the calibration sensor is the sameas an impedance of an active sensor of the plurality of active sensors,and an electrical carrier count of the calibration sensor is greaterthan an electrical carrier count of the active sensor.

In some embodiments, the sensor circuit further includes: a readoutelement corresponding to an active sensor of the plurality of activesensors and configured to measure a readout voltage of the activesensor, the calibration readout element is configured to measure areadout voltage of the calibration sensor, and the sensor circuit iselectrically coupled to: a processor; and a memory includinginstructions, which when executed by the processor, cause the processorto perform a method that includes: receiving the readout voltage of theactive sensor; receiving the readout voltage of the calibration sensor;and computing a difference between (1) the readout voltage of the activesensor and (2) the readout voltage of the calibration sensor weighted bya ratio between an impedance of the calibration sensor and an impedanceof the active sensor.

In some embodiments, the ratio is one.

In some embodiments, the ratio is temperature independent.

In some embodiments, the sensor circuit further includes: a readoutelement corresponding to an active sensor of the plurality of activesensors and configured to measure a readout voltage of the activesensor, the sensor circuit is electrically coupled to a processor and amemory including instructions, which when executed by the processor,cause the one or more processors to perform a method that includes:receiving a first readout voltage corresponding to a closed shutter;receiving a second readout voltage corresponding to an opened shutter;and computing a difference proportional to an impedance difference ofthe active sensor caused by the sensor image between (1) the firstreadout voltage and (2) the second readout voltage.

In some embodiments, the plurality of readout elements includes aplurality of ADCs.

In some embodiments, the calibration sensor and the plurality of activesensors are made from materials having a same thermal coefficient ofresistance (TCR).

In some embodiments, the plurality of active sensors includes aplurality of columns of active sensors, the circuit further includes: aplurality of current sources, a current source of the plurality ofcurrent sources is electrically coupled to the second terminal of thecalibration sensor and the calibration readout element; and a pluralityof readout elements, each of the plurality of columns of active sensorsis electrically coupled to: a corresponding current source of theplurality of current sources at a corresponding readout node, and acorresponding readout element of the plurality of readout elements atthe corresponding readout node.

In some embodiments, the calibration readout element includes ananalog-to-digital converter (ADC).

In some embodiments, the plurality of active sensors and the calibrationsensor are bolometers, and the sensor image is a thermal image.

In some embodiments, a sensor circuit includes: a calibration currentsource providing a calibration current; an active sensor; a readoutelement; a first switch configured to selectively electrically couplethe active sensor to the readout element; and a second switch configuredto selectively electrically couple the calibration current source to thereadout element.

In some embodiments, the second switch is configured to electricallyuncouple the calibration current source from the first readout elementwhen the first switch electrically couples the active sensor to thereadout element, and the first switch is configured to electricallyuncouple the active sensor from the first readout element when thesecond switch electrically couples the calibration current to thereadout element, and the sensor circuit is electrically coupled to: aprocessor; and a memory including instructions, which when executed bythe processor, cause the processor to perform a method including:receiving a first readout voltage of the active sensor; receiving asecond readout voltage caused by the calibration current; and computingan output proportional to a readout current of the active sensor basedon (1) the first readout voltage and (2) the second readout voltage.

In some embodiments, the sensor circuit further includes: a plurality ofactive sensors including the active sensor; and a plurality of readoutelements including the first and second readout elements, each of theplurality of readout elements electrically coupled to a respectiveactive sensor of the plurality of active sensors, the method furtherincludes: receiving, from a readout element of the plurality of readoutelements, a first readout voltage of the respective active sensor;receiving a second readout voltage caused by the calibration current ona respective readout element; and computing a respective outputproportional to a readout current of the respective active sensor basedon (1) the readout voltage of the respective sensor and (2) the secondreadout voltage caused by the calibration current on the respectivereadout element.

In some embodiments, the sensor circuit further includes a second activesensor belonging to a same column as the first active sensor, the methodfurther includes, after computing the first output: receiving a thirdreadout voltage of the second active sensor; and computing a secondoutput proportional to a readout current of the fourth active sensorbased on (1) the third readout voltage and (2) the second readoutvoltage caused by the calibration current.

In some embodiments, a time between successive receipts of the secondreadout voltage on the same column caused by the calibration current isa calibration period.

In some embodiments, the calibration period is one second.

In some embodiments, the calibration period is based on a drift of thereadout element.

In some embodiments, different rows are readout during the successivereceipts of the second readout voltage.

In some embodiments, the sensor circuit further includes: a secondcalibration current source; a third switch configured to selectivelyelectrically couple the first calibration current source to the readoutelement; and a fourth switch configured to selectively electricallycouple the second calibration current source to the readout element, andwhen the third switch electrically uncouples the readout element fromthe first calibration current source: the fourth switch is configured toelectrically couple the readout element to the second calibrationcurrent source, and the method further includes receiving a thirdreadout voltage caused by the second calibration current; and the outputis further based on the third readout voltage caused by the secondcalibration current.

In some embodiments, the readout element includes an ADC.

In some embodiments, the sensor circuit is electrically coupled to: aprocessor; and a memory including instructions, which when executed bythe processor, cause the one or more processors to perform a method thatincludes: receiving a first readout voltage corresponding to a closedshutter; receiving a second readout voltage corresponding to an openedshutter; and computing a difference proportional to an impedancedifference of the first active sensor caused by a sensor image between(1) the first readout voltage and (2) the second readout voltage.

In some embodiments, the active sensor is a bolometer exposed to athermal scene.

In some embodiments, the active sensor is exposed to a sensor image andshares a bias voltage node with a plurality of active sensors, and thesensor circuit further includes: a second readout element; and acalibration sensor shielded from the sensor image and including a firstterminal electrically coupled to the bias voltage node and a secondterminal electrically coupled to the second readout element.

Some embodiments include a method of manufacturing the above circuits.

In some embodiments, a method of calculating a calibrated voltage in asensor circuit includes: electrically coupling a first terminal of acalibration sensor to a bias voltage node shared by a plurality ofactive sensors; electrically coupling a second terminal of thecalibration sensor to a calibration readout element; exposing theplurality of active sensors to a sensor image; shielding the calibrationsensor from the sensor image; measuring, with a readout element, areadout voltage of an active sensor of the plurality of active sensors;measuring, with the calibration readout element, a readout voltage ofthe calibration sensor; and computing the calibrated voltage as adifference between (1) the readout voltage of the active sensor and (2)the readout voltage of the calibration sensor weighted by a ratiobetween an impedance of the calibration sensor and an impedance of theactive sensor.

In some embodiments, the impedance of the calibration sensor is the sameas the impedance of the active sensor, and an electrical carrier countof the calibration sensor is greater than an electrical carrier count ofthe active sensor.

In some embodiments, the ratio is one.

In some embodiments, the ratio is temperature independent.

In some embodiments, the calibration sensor and the active sensor aremade from materials having a same TCR.

In some embodiments, the method further includes: electrically couplinga current source of a plurality of current sources to the secondterminal of the calibration sensor and to the calibration readoutelement; electrically coupling a column of a plurality of columns ofactive sensors to the readout element, the column of active sensorsincluding the active sensor; and electrically coupling a second currentsource of the plurality of current sources to the readout element.

In some embodiments, the method further includes: closing a shutter;measuring, with the readout element, a first readout voltagecorresponding to the closed shutter; and measuring, with the calibrationreadout element, a second readout voltage corresponding to the closedshutter; and after computing the calibrated voltage, computing a seconddifference between (1) the calibrated voltage and a difference between(2a) the first readout voltage and (2b) the second readout voltageweighted by the ratio, the second difference is a shutter calibratedvoltage.

In some embodiments, the calibration readout element includes an ADC.

In some embodiments, the readout element includes an ADC.

In some embodiments, the plurality of active sensors and the calibrationsensor are bolometers, and the sensor image is a thermal image.

In some embodiments, a method of calculating an output in a sensorcircuit includes: electrically coupling a readout element to an activesensor; measuring, with the readout element, a first readout voltage ofthe active sensor; electrically uncoupling the readout element from theactive sensor; electrically coupling a calibration current to thereadout element; measuring, with the readout element, a second readoutvoltage caused by the calibration current; and computing the outputbased on (1) the first readout voltage and (2) the second readoutvoltage, the output proportional to a readout current of the activesensor.

In some embodiments, the method further includes: electrically couplinga respective active sensor of a plurality of active sensors to a readoutelement of a plurality of readout elements; measuring, with therespective readout element, a first readout voltage of the respectiveactive sensor; electrically uncoupling the respective readout elementfrom the respective active sensor; electrically coupling the calibrationcurrent to the respective readout element; measuring, with therespective readout element, a second readout voltage caused by thecalibration current on the respective readout element; and computing anoutput proportional to a readout current of the respective active sensorbased on (1) the first readout voltage of the respective active sensorand (2) the second readout voltage caused by the calibration current.

In some embodiments, the method further includes, after computing thefirst output: electrically uncoupling the calibration current sourcefrom the readout element; electrically coupling the readout element to asecond active sensor, the second active sensor belonging to a samecolumn as the first active sensor; measuring, with the readout element,a third readout voltage of the second active sensor; and computing asecond output proportional to a readout current of the second activesensor based on (1) the third readout voltage and (2) the second readoutvoltage caused by the calibration current.

In some embodiments, a time between successive measurements of thesecond readout voltage on the same column caused by the calibrationcurrent is a calibration period.

In some embodiments, the calibration period is one second.

In some embodiments, the calibration period is based on a drift of thereadout element.

In some embodiments, different rows are readout during the successivemeasurements of the second readout voltage.

In some embodiments, the method further includes: electricallyuncoupling the readout element from the first calibration currentsource; electrically coupling the readout element to a secondcalibration current source; and measuring, with the readout element, athird readout voltage caused by the second calibration current on thereadout element, the output is further based on the third readoutvoltage caused by the second calibration current.

In some embodiments, the readout element includes an ADC.

In some embodiments, the method further includes: closing a shutter;computing the output corresponding to a closed shutter; and computing adifference proportional to an impedance difference of the active sensorcaused by a sensor image between (1) the output corresponding to anopened shutter and (2) the output corresponding to the closed shutter.

In some embodiments, the active sensor is a bolometer exposed to athermal scene.

In some embodiments, the method further includes: electricallyuncoupling the readout element from the calibration current source;electrically coupling a second readout element to the calibrationcurrent source; measuring, with the second readout element, a thirdreadout voltage caused by the calibration current; electricallyuncoupling the second readout element from the calibration currentsource; electrically coupling a first terminal of a calibration sensorto a bias voltage node shared by a plurality of active sensors and theactive sensor; electrically coupling a second terminal of thecalibration sensor to the second readout element; exposing the pluralityof active sensors and the active sensor to a sensor image; shielding thecalibration sensor from the sensor image; measuring, with the secondreadout element, a fourth readout voltage of the calibration sensor;computing a second output based on the third readout voltage and thefourth readout voltage; and computing a difference between (1) the firstoutput and (2) the second output weighted by a ratio between animpedance of the calibration sensor and an impedance of the activesensor.

Examples of the disclosure are directed toward sensor circuits andmethods that overcome the herein-identified drawbacks (e.g., powerchallenges, area challenges). In some embodiments, the sensor circuitincludes a plurality of sensor pixels, a Sigma-Delta ADC, and aplurality of switches. In some embodiments, the sensor circuit includescolumns of sensors, and different parts of a column of sensors are beingread out at a same time.

In some embodiments, a sensor circuit, includes: a plurality of sensorpixels, each configured to store a charge; a Sigma-Delta ADC configuredto receive the charge of each sensor; and a plurality of switchesconfigured to sequentially couple each of the plurality of sensor pixelsto the Sigma-Delta ADC, each switch corresponding to a respective one ofthe plurality of sensor pixels.

In some embodiments, the sensor circuit does not include a CTIAelectrically positioned between the plurality of sensor pixels and theSigma-Delta ADC.

In some embodiments, the sensor circuit further includes a variableresistor electrically positioned between the plurality of sensors andthe Sigma-Delta ADC, wherein the plurality of switches are configured tosequentially couple each of the plurality of sensor pixels to thevariable resistor.

In some embodiments, the variable resistor has a linearly decreasingresistance during a discharge time window; the variable resistor is at alowest resistance at an end of the discharge time window; and thevariable resistor has a resistance higher than the lowest resistancebetween the beginning and the end of the discharge time window.

In some embodiments, the variable resistor is a MOS transistor; and theinitial resistance, the linearly decreasing resistance, and the lowestresistance of the MOS transistor are controlled with a control voltageelectrically coupled to the MOS transistor.

In some embodiments, the discharge time window is between 10microseconds and 1 millisecond.

In some embodiments, during the first discharge time window, a firstswitch electrically couples a first sensor pixel and the Sigma-DeltaADC; during a second discharge time window, a second switch electricallycouples a second sensor pixel and the Sigma-Delta ADC; and the first andsecond discharge time windows correspond to readout times of the firstand second sensor pixels.

In some embodiments, during the discharge time window, a constantcurrent of the variable resistor is an initial voltage of the variableresistor divided by the initial resistance.

In some embodiments, a switch electrically couples a respective sensorpixel and the variable resistor during a respective discharge timewindow, the discharge time window equal to a capacitance of the sensorpixel multiplied by an initial resistance of the variable resistor.

In some embodiments, the variable resistor includes a weighted bank ofresistors; the weighted bank of resistors include a plurality ofresistors selectively electrically coupled in parallel or in series; andresistances of combinations of the selective electrically coupledresistors include an initial resistance at the beginning of a dischargetime window, a linearly decreasing resistance, and a lowest resistance.

In some embodiments, a sensor pixel includes an x-ray sensor photodiodeand the charge is indicative of the x-ray sensor photodiode's exposureto x-ray.

In some embodiments, a sensor pixel includes a storage capacitor storingthe charge and the sensor pixel's exposure to x-ray generates the chargestored in the storage capacitor.

In some embodiments, the sensor circuit further includes a secondplurality of sensor pixels and a second Sigma-Delta ADC, wherein thesecond plurality of sensor pixels are configured to sequentially coupleto the second Sigma-Delta ADC and the first and second pluralities ofsensor pixels belong to a same column.

In some embodiments, numbers of the first and second plurality of sensorpixels are equal.

In some embodiments, at a first row time, a first sensor pixel of thefirst plurality of sensor pixels and a second sensor pixel of the secondplurality of sensor pixels are simultaneously readout.

In some embodiments, an input current to the Sigma-Delta ADC isconstant.

In some embodiments, the sensor circuit further includes a digitalfilter configured to receive a signal from the Sigma-Delta ADC.

Some embodiments include a method of manufacturing the above circuits.

In some embodiments, a sensor circuit includes a plurality of sensorpixels, a Sigma-Delta ADC, and a plurality of switches, each switchcorresponding to a respective one of the plurality of sensor pixels; amethod of readout of the sensor circuit includes: storing respectivecharges in each of the plurality of sensor pixels; sequentiallyelectrically coupling, using the plurality of switches, each of theplurality of sensor pixels to the Sigma-Delta ADC; and sequentiallyreceiving, at the Sigma-Delta ADC, the respective charge of each sensorpixel.

In some embodiments, the sensor circuit does not include a CTIAelectrically positioned between the plurality of sensor pixels and theSigma-Delta ADC and the respective charge of each sensor pixel is notreceived by the CTIA.

In some embodiments, the sensor circuit further includes a variableresistor electrically positioned between the plurality of sensor pixelsand the Sigma-Delta ADC and the method further includes sequentiallyelectrically coupling, using the plurality of switches, each of theplurality of sensor pixels to the Sigma-Delta ADC further includessequentially electrically coupling, using the plurality of switches, theeach of the plurality of sensor pixels to the variable resistor.

In some embodiments, the method further includes linearly decreasing aresistance of the variable resistor during a discharge time window,wherein: the variable resistor is at a lowest resistance at an end ofthe discharge time window; and the variable resistor has a resistancehigher than the lowest resistance between the beginning and the end ofthe discharge time window.

In some embodiments, the variable resistor is a MOS transistorelectrically coupled to a control voltage and linearly decreasing theresistance of the variable resistor further includes driving the MOStransistor with the control voltage to generate the initial resistance,the linearly decreasing resistance, and the lowest resistance.

In some embodiments, the discharge time window is between 10microseconds and 1 millisecond.

In some embodiments, sequentially electrically coupling, using theplurality of switches, each of the plurality of sensor pixels to theSigma-Delta ADC further includes: during the first discharge timewindow, electrically coupling a first switch to a first sensor pixel andthe Sigma-Delta ADC; during a second discharge time window, electricallycoupling a second switch to a second sensor pixel and the Sigma-DeltaADC, wherein the first and second discharge time windows correspond toreadout times of the first and second sensor pixels.

In some embodiments, during the discharge time window, a constantcurrent of the variable resistor is an initial voltage of the variableresistor divided by the initial resistance.

In some embodiments, sequentially electrically coupling, using theplurality of switches, each of the plurality of sensor pixels to theSigma-Delta ADC further includes electrically coupling a switch to arespective sensor pixel and the variable resistor during a respectivedischarge time window; and the discharge time window is equal to acapacitance of the sensor pixel multiplied by an initial resistance ofthe variable resistor.

In some embodiments, the variable resistor includes a weighted bank ofresistors; the weighted bank of resistors include a plurality ofresistors selectively electrically coupled in parallel or in series; andthe method further includes linearly decreasing resistances ofcombinations of the plurality of resistors, from an initial resistanceat a beginning of a discharge time window to a lowest resistance at anend of the discharge time window, by selective electrically coupling theresistors.

In some embodiments, a sensor pixel includes an x-ray sensor photodiodeand the charge is indicative of the x-ray sensor photodiode's exposureto x-ray.

In some embodiments, storing respective charges in each of the pluralityof sensor pixels further includes: exposing the each of the plurality ofsensor pixels to x-ray and generating the respective charge; and storingthe respective charges in a storage capacitor of the each of theplurality of sensor pixels.

In some embodiments, the sensor circuit further includes a secondplurality of sensor pixels belonging to a same column as the firstplurality of sensor pixels, a second plurality of switches, and a secondSigma-Delta ADC, the method further includes: sequentially electricallycoupling, using the second plurality of switches, each of the pluralityof sensor pixels to the second Sigma-Delta ADC; and sequentiallyreceiving, at the second Sigma-Delta ADC, the respective charge of eachsensor pixel of the second plurality of sensor pixels.

In some embodiments, numbers of the first and second plurality of sensorpixels are equal.

In some embodiments, at a first row time: the first Sigma-Delta ADCreceives a first respective charge of a first sensor pixel of the firstplurality of sensor pixels; and the second Sigma-Delta ADC receives asecond respective charge of a second sensor pixel of the secondplurality of sensor pixels.

In some embodiments, the Sigma-Delta ADC receives a constant current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a sensor readout circuit, in accordance with anembodiment.

FIG. 2 illustrates a method of sensor readout, in accordance with anembodiment.

FIG. 3 illustrates a sensor readout circuit, in accordance with anembodiment.

FIG. 4 illustrates a method of sensor readout, in accordance with anembodiment.

FIG. 5 illustrates a sensor readout circuit, in accordance with anembodiment.

FIG. 6 illustrates a sensor readout circuit, in accordance with anembodiment.

FIG. 7 illustrates a method of current adjustment, in accordance with anembodiment.

FIG. 8 illustrates a sensor bias circuit, in accordance with anembodiment.

FIG. 9 illustrates a sensor readout system, in accordance with anembodiment.

FIG. 10 illustrates a sensor readout circuit, in accordance with anembodiment.

FIG. 11 illustrates a sensor calibration circuit, in accordance with anembodiment.

FIG. 12 illustrates a method of sensor calibration according to examplesof the disclosure.

FIG. 13 illustrates a sensor calibration circuit, in accordance with anembodiment.

FIG. 14 illustrates a method of sensor calibration, in accordance withan embodiment.

FIG. 15 illustrates a sensor calibration circuit, in accordance with anembodiment.

FIG. 16 illustrates a sensor calibration circuit, in accordance with anembodiment.

FIG. 17 illustrates a method of sensor calibration, in accordance withan embodiment.

FIGS. 18A and 18B illustrate exemplary sensor images, in accordance withan embodiment.

FIGS. 19A and 19B illustrate exemplary sensor circuits, in accordancewith an embodiment.

FIGS. 20A and 20B illustrate exemplary sensor circuits, in accordancewith an embodiment.

FIGS. 21A to 21D illustrate exemplary inputs, in accordance with anembodiment.

FIG. 22 illustrates a method of manufacturing MEMS products, inaccordance with an embodiment.

FIG. 23 illustrates a bolometer, in accordance with an embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following description of embodiments, reference is made to theaccompanying drawings which form a part hereof, and in which it is shownby way of illustration specific embodiments which can be practiced. Itis to be understood that other embodiments can be used and structuralchanges can be made without departing from the scope of the disclosedembodiments.

Examples of the disclosure are directed toward MEMS sensor readoutcircuits and methods that overcome the herein-identified drawbacks. Insome embodiments, sensor readout circuits include a reference sensor, anactive sensor, current sources, a voltage driver, and a readout element.In some embodiments, methods include providing a current to a referencesensor, generating a common mode-tracking bias voltage electricallycoupled to an active sensor, and measuring a current change at a readoutelement.

As an exemplary advantage, the disclosed circuits and methods reducecommon mode effects without the cost and complexity of additionalcircuitry. The circuits presented herein can efficiently and compactlytrack common mode changes in the sensor array. Thus, accuracy of themeasured thermal image signal can be improved, and the input rangerequirement of subsequent analog-to-digital converters (ADC) can bereduced without additional components for compensation. By trackingcommon mode changes with a bias voltage, speed, and accuracy can beimproved because column voltages can be nominally at fixed voltages andunaffected by column parasitic elements.

FIG. 1 illustrates a sensor readout circuit 100, in accordance with anembodiment. The sensor readout circuit 100 includes a readout element102, a first current source 104, a second current source 106, a voltagedriver 108, a reference sensor 110, and an active sensor 112. Someembodiments include a method of manufacturing the readout circuit 100.

The readout element includes an input 103. The voltage driver includesan output 109. The reference sensor 110 includes a first terminal 110 aand a second terminal 110 b; the first terminal 110 a is electricallycoupled to the first current source 104 and the second terminal 110 b iselectrically coupled to the output 109 of the voltage driver 108. Theactive sensor 112 includes a first terminal 112 a and a second terminal112 b; the first terminal 112 a is electrically coupled to the secondcurrent source 106 and the input 103 of the readout element 102; and thesecond terminal 112 b is electrically coupled to the output 109 of thevoltage driver 108. The active sensor 112 is configured for exposure toa sensor image.

In some embodiments, a voltage follower (not shown) can be electricallycoupled between output 109 and second terminal 112 b. A voltage followermay advantageously buffer the voltage at output 109 from effects ofcapacitive loading.

The simplified topology, as exemplified in FIG. 1, requires one voltagedriver, which provides a bias voltage. Without adding circuitry toreduce undesired common mode effects, the topology can reduce undesiredcommon effects without increasing complexity, size, and cost,introducing more parasitics and unknowns, consuming more power, andadding noise inducing components.

In some embodiments, the reference sensor 110 is shielded from thesensor image. The reference sensor 110 can be exposed to an ambientcondition common to the reference sensor and active sensor. For example,the reference sensor is a reference or blind bolometer pixel that isexposed to ambient temperatures, but is not exposed to a thermal scene,and the active sensor is an active bolometer pixel, which is exposed toboth the ambient temperatures and the thermal scene. Generally, thesensors include two terminals and can have a variable impedance valuebetween the two terminals that depends on a sensor image. Althoughresistor and variable resistor symbols are used to represent the sensorsin this disclosure, it is understood that sensor properties andcomponents are not limited to resistive elements between two terminals.

Although the term “bolometer” is used to exemplify the disclosedsensors, it is understood that the term “bolometer” is not limited to asingle pixel or a single device. A bolometer can be any element that isconfigured to change an output characteristic in response to exposedradiation. In some examples, a bolometer can be one or more pixels. Inother examples, a bolometer can be one more devices. Although the term“bolometer pixel” is used to exemplify the disclosed sensors, it isunderstood that the disclosed sensors can include more than onebolometer pixel without departing from the scope of this disclosure.

In some embodiments, the reference sensor 110 generates an adjustablebias voltage at output 109 that tracks both ambient conditions andself-heating. In such examples, the readout circuit operates in aconstant current mode, compared to the constant voltage mode of otherreadout circuits. In other words, a first current provided by the firstcurrent source 104 is constant. Since no other branch is connectedbetween the first current source and the reference sensor, the currenttraversing the reference sensor is also constant at the value of thefirst current.

Because the reference sensor is exposed to ambient conditions, theimpedance of the reference sensor stabilizes according to a value thatis reflective of the exposed ambient conditions. Although the impedanceof the sensor is substantially fixed at a stable state, it is understoodthat the impedance of the reference sensor can vary according to theambient conditions. A voltage drop is generated between first terminal110 a and second terminal 110 b due to the first current and thereference sensor impedance. Due to the voltage drop, the voltage at thesecond terminal 110 b is generated by the difference between the voltageat the first terminal 110 a and the voltage drop across the twoterminals.

The voltage at the second terminal 110 b is driven by voltage driver 108at output 109. In an embodiment, voltage driver 108 can actsubstantially as an ideal voltage source. In other words, the voltagedriver 108 can provide (or absorb) the necessary current at output 109to maintain the generated voltage at the second terminal 110 b.

The symbol illustrating voltage driver 108 is used for illustrativepurposes only. It is apparent to a person of ordinary skill in the artthat different methods and circuits can be utilized to drive the voltageat the second terminal 110 b. Although an input of the illustrativevoltage driver 108 is shown as floating, it is understood that the inputis merely representative and can be connected to a suitable element ofthe readout circuit to maintain the output voltage.

The voltage at the second terminal 110 b is significant because it isreflective of common mode effects, such as self-heating and ambientconditions, observed by the reference sensor 110. By driving the secondterminal 112 b of active sensor 112 and biasing it at this voltage, thecommon mode effects (which can disadvantageously reduce the dynamicrange of subsequent stages) have been effectively compensated becausethe biasing voltage is reflective of common mode conditions.

In some embodiments, the sensor readout circuit 100 is symmetricallyconstructed. In other words, active sensor 112 is substantially the sameas reference sensor 110, and the second current source 106 can providesubstantially the same current as the first current source 104.

In some embodiments, the first current source and the second currentsource are configured to output an equal magnitude of current in a samedirection relative to the respective first terminals. In an example,currents provided by the first and second current sources can flowtoward a reference potential 120. In some embodiments, the referencepotential 120 is at a reference voltage. In another embodiment, thereference potential 120 is a ground.

In some examples, input 103 of the readout element 102 can have a lowinput resistance, such as an input of a charge amplifier. In theseexamples, if the reference and active sensors are exposed tosubstantially the same conditions, and the first and second currentsources provide substantially the same currents, no current would enteror exit the input 103 because the current traversing the active sensor112 would be the same as the current provided by the second currentsource 106. An example of this condition is when the exposed sensorimage is the ambient condition itself.

Conversely, if the reference and active sensors are exposed to differentconditions (i.e. the exposed sensor image is different than the ambientconditions), then a current would enter or exit the input 103 becausethe impedance of the active sensor 112 is different from the impedanceof the reference sensor 110, and the values of the provided currentsremain the same after exposure. The current or total charge entering orexiting the input 103 is captured and measured in the readout element102.

Since common mode effects such as self-heating and ambient conditionshave been compensated by the bias voltage, the measured current orcharge is independent of these common mode components. As a result, thedynamic range of subsequent stages can be reduced.

Since the measured current or charge is independent of undesired commonmode components, these common mode components are not part of themeasurement. As an exemplary advantage, the disclosed circuits andmethods remove common mode effects without the cost and complexity ofadditional circuitry. The circuits presented herein efficiently andcompactly track common mode changes in the sensor array. Thus, accuracyof the measured thermal image signal can be improved and the input rangerequirement of subsequent ADCs can be reduced without additionalcomponents for compensation.

Additionally, the disclosed readout circuits and methods can tolerategreater variations of circuit elements. For example, since the commonmode effects are removed, thermal dependency of the transistors andresistive components can be ignored.

Utilizing this mechanism, an array of active sensors can be measured.Based on the captured or measured currents or charges, a sensor imagecan be computed. Exemplary methods and circuits performing themeasurements and computations are discussed later in this disclosure.

In an embodiment, the active sensors are active bolometer pixels exposedto a thermal scene, and the reference sensors are blind or referencebolometer pixels that are exposed to ambient temperatures, but not thethermal scene. In some embodiments, the active bolometer pixel isexposed to LWIR radiation. The reference bolometer pixel determines abias voltage that compensates for common mode effects. In response tochanges in the thermal scene, the impedance of the active bolometerpixel can change and the resulting change in current is measured orcaptured to determine the thermal image associated with the thermalscene. The measured current is compensated for common mode effects.

In some embodiments, the first current and the second current providedby the current sources 104 and 106 are constant. For example, thecurrent sources are athermal. The values of the currents aresubstantially unaffected by temperature. Additionally or alternatively,the current sources can substantially act like an ideal current source.For example, a value of current provided by the current source is fixedregardless of an output voltage of the corresponding current source. Asused herein, “constant current” means a current that is independent ofother parameters (e.g., temperature, output voltage of a current source,driving load, driving speed). A person of ordinary skill in the art willunderstand that “constant current” does not require a same value at alltimes. For example, the current can have a first fixed value during areadout time and have a second fixed value during a non-readout period(e.g., calibration, sleep, low power, power off). In another example,the current can have a third fixed value during a first readout time andhave a fourth fixed value during a second readout time.

In some embodiments, a first current buffer is coupled between thereference sensors and the first current source and a second currentbuffer is coupled between the active sensors and the input of thereadout element. In some embodiments, the current buffers shieldundesired effects at the input nodes (e.g., input of the readoutelement, an input of the voltage driver) from signals generated by thereference and active sensors. For example, undesired current can beinjected into the input nodes and modulate the voltages at these nodes.The current buffers prevent the signal currents from being affected bythe undesired injected currents.

Although current source symbols are used to illustrate the currentsources in this disclosure, it is understood that the current source canbe any one or more circuit elements that can provide a constant and/orathermal current. In some embodiments, the first current source and thesecond current source are one or more of an athermal voltage source andresistor, a high-impedance athermal transistor current source, and aWilson current mirror.

As an exemplary advantage, since constant and athermal current sourcescan be used to provide the currents of the readout circuit, the outputimpedance of the current sources can be high compared to the sensors.Thus, the efficiency of the circuit increases because more signalcurrent enters the readout element, instead of being divided by aresistive divider formed by reference and active sensors.

In some embodiments, the readout circuit is electrically coupled to acalibration circuit. The calibration circuit includes one or morecalibration current sources, one or more fourth reference sensors, and acalibration readout element. The one or more calibration current sourcesare configured to provide one or more calibration currents toelectrically coupled columns of active and reference sensors and thecalibration readout element. In some embodiments, the calibrationcurrent sources are fixed current sources. The one or more fourthreference sensors are electrically coupled to the bias voltage and thecalibration readout element. In some embodiments, the calibrationreadout element is a calibration ADC. In some embodiments, each of theone or more fourth references is physically larger than a referencesensor of the readout circuit and has a same impedance as a referencesensor of the readout circuit. The physically larger fourth referencesensor is less noisy than a reference sensor of the readout circuit.

In some embodiments, the readout element is an ADC; an ADC is associatedwith a column of active or reference sensors. Non-idealities such asgain mismatches, noise, and/or offset of the ADC are calibrated in acalibration mode. In some examples, the non-idealities cause undesiredfixed patterns, distorting the sensor image. In the calibration mode,during readout of each row, some or all the voltage driver and readoutelement inputs are electrically decoupled from the reference and activesensors of the readout circuit, respectively. One or more calibrationcurrents are electrically coupled to the voltage driver and readoutelement inputs that are electrically decoupled from the reference andactive sensors of the readout circuit and the calibration readoutelement. During readout of each row, a measured value of the calibrationreadout element can be subtracted from one or more measured values ofthe readout element inputs electrically coupled to the one or morecalibration currents. In some embodiments, the one or more calibrationcurrents electrically couple to one or more different readout elementinputs for each row readout period.

Although “electrically coupled” and “coupled” are used to describe theelectrical connections between two elements of the readout circuit inthis disclosure, it is understood that the electrical connections do notnecessarily need direct connection between the terminals of thecomponents being coupled together. Different combinations andconnections of the recited components can achieve a constant current andadjustable bias voltage readout circuit without departing from the scopeof this disclosure. For example, electrical routing connects between theterminals of the components being electrically coupled together. Inanother example, a closed (conducting) switch is connected between theterminals of the components being coupled together. In yet anotherexample, additional elements connect between the terminals of thecomponents being coupled together without affecting the constant currentcharacteristics of the circuit. For example, buffers, amplifiers, andpassive circuit elements can be added without affecting thecharacteristics of the readout circuit and departing from the scope ofthis disclosure.

In some embodiments, two electrically coupled components may betopologically coupled. As used herein, two components are “topologicallycoupled” if they provide an electrical influence on one another within atopology or a same part of a topology. For example, the reference sensorand the first current source of the disclosed readout circuits areelectrically coupled on a same reference branch of the readout circuit.

Similarly, although “electrically uncoupled” is used to describeelectrical disconnects between two elements of the readout circuit inthis disclosure, it is understood that electrical disconnects do notnecessarily need to be physically open between the terminals of thecomponents being switched. It is also understood that “uncoupled” is notlimited to mean prevention of electrical energy transfer between twoelements. For example, high-impedance elements are connected between theterminals of the components being uncoupled. In another example, anopened (non-conducting) switch is connected between the terminals of thecomponents being uncoupled, effectively uncoupling the components.

The arrows used in the figures are for illustrative purposes. It isunderstood that that the direction of current flow is not limited to thedirection indicated on the drawings. For the sake of descriptiveness,terms such as “entering” and “exiting” are used to describe currentflow. A person of ordinary skill in the art would recognize that thedirections of current flow or the polarities of the voltages are notlimited to the directions or polarities described or illustrated. Insome embodiments, voltage polarity is determined by current directionand effective impedance of components traversed by the currents. Inother embodiments, current direction is determined by voltage polarityand effective impedance of components traversed by the currents. In yetother embodiments, the current directions are determined by currentsources and the voltage polarities are determined by voltage sources.

FIG. 2 illustrates a method 200 of sensor readout, in accordance with anembodiment. Method 200 includes providing a first current to a firstterminal of a reference sensor (step 202). For example, referring toFIG. 1, a first current source 104 can provide a first current to thefirst terminal 110 a of the reference sensor 110.

Method 200 includes generating, from the first current, a voltage at asecond terminal of the reference sensor (step 204). For example, due tothe first current and the impedance of the reference sensor, a voltagedrop across the reference sensor 110 and a voltage at the secondterminal 110 b are generated.

Method 200 includes providing a second current to a first terminal ofthe active sensor (step 206). For example, a second current source 106can provide a second current to the first terminal 112 a of the activesensor 112.

In some embodiments, the first current and the second current areconstant. In some embodiments, the first current source and the secondcurrent source are configured to output an equal magnitude of current ina same direction relative to the respective first terminals. Forexample, currents provided by the first current source 104 and secondcurrent source 106 can flow toward a reference potential 120. In someembodiments, the reference potential 120 is a reference voltage drivenby a voltage source. In another embodiment, the reference potential 120is a ground.

In some embodiments, the first current source and the second currentsource are selected from the group of an athermal voltage source andresistor, a high-impedance athermal transistor current source, and aWilson current mirror.

Method 200 includes driving, at the voltage, a second terminal of anactive sensor (step 208). For example, the second terminal 110 b of thereference sensor 110 is electrically coupled to the second terminal 112b of the active sensor 112, and the voltage at this node is driven byvoltage driver 108 at output 109. In some embodiments, the voltage is abias voltage for the active sensor. For example, as described earlier,the bias voltage is reflective of common mode effects. By biasing theactive sensor at the voltage, these common mode effects can becompensated.

In some embodiments, a voltage follower can be electrically coupledbetween output 109 and second terminal 112 b. A voltage follower mayadvantageously buffer the voltage at output 109 from effects ofcapacitive loading.

Method 200 includes exposing the active sensor to a sensor image (step210). For example, active sensor 112 is exposed to a sensor image. Insome embodiments, exposing the active sensor to the sensor image furtherincludes changing an impedance of the active sensor. For example, theactive sensor is an active bolometer pixel. The active bolometer pixelis exposed to a thermal scene. The impedance of the active bolometerpixel can change in response to exposure to the thermal scene. In someembodiments, exposing the active sensor to the sensor image includesexposing the active sensor to LWIR radiation.

Method 200 includes measuring a third current from the first terminal ofthe active sensor to an input of a readout element (step 212). Forexample, a current entering or exiting input 103 of the readout element102 is measured.

In some embodiments, exposing the active sensor to the sensor imagefurther includes changing the third current. For example, the activesensor is an active bolometer pixel. The active bolometer pixel isexposed to a thermal scene. In response to exposure to the thermalscene, a current entering or exiting input 103 of the readout element102 can change.

In some embodiments, method 200 further includes exposing the referencesensor to an ambient condition common to the reference sensor and activesensor; and shielding the reference sensor from the sensor image. In oneexample, the sensor is a bolometer pixel and the common conditions areself-heating and ambient temperature. The reference bolometer pixel isexposed to the common conditions, but is shielded from the thermalscene.

In some embodiments of method 200, the reference sensor is a referencebolometer pixel and the active sensor is an active bolometer pixel. Inan embodiment, the active sensor is an active bolometer pixel exposed toa thermal scene, and the reference sensor is a blind or referencebolometer pixel that is exposed to ambient temperatures, but not thethermal scene. The reference bolometer pixel can determine a biasvoltage that compensates for common mode effects. In response to changesof the thermal scene, the impedance of the active bolometer pixel canchange and the resulting change in current is measured or captured todetermine the thermal image associated with the thermal scene. Themeasured current is compensated for common mode effects.

In some embodiments, a first current buffer is coupled between thereference sensors and the first current source and a second currentbuffer is coupled between the active sensors and the input of thereadout element. In some embodiments, the current buffers shieldundesired effects at the input nodes (e.g., input of the readoutelement, an input of the voltage driver) from signals generated by thereference and active sensors. For example, undesired current can beinjected into the input nodes and modulate the voltages at these nodes.The current buffers prevent the signal currents from being affected bythe undesired injected currents.

FIG. 3 illustrates a sensor readout circuit, in accordance with anembodiment. The readout circuit 300 includes components that aresubstantially similar to those described in FIG. 1. Like components inFIG. 3 are given like numerals as the corresponding components in FIG. 1(for example, read out element 102 and read out element 302). For thesake of brevity, those like components are not described again withrespect to FIG. 3. Some embodiments include a method of manufacturingthe readout circuit 300.

Circuit 300 further includes a second reference sensor 320, a secondactive sensor 322, a first switch 330, a second switch 332, a thirdswitch 334, a fourth switch 336, and a voltage follower 318. The secondreference sensor 320 includes a first terminal 320 a and a secondterminal 320 b; the first terminal 320 a is electrically coupled to thefirst current source 304 and the second terminal 320 b is electricallycoupled to output 309 of voltage driver 308. The second active sensor322 includes a first terminal 322 a and a second terminal 322 b; thefirst terminal 322 a is electrically coupled to the second currentsource 306 outputting the second current and the second terminal 322 bis electrically coupled to voltage follower 318. The second activesensor 322 is configured to change the current from the first terminal322 to the input 303 of the readout element 302.

A voltage follower 318 is electrically coupled between the output 309and second terminal 312 b of the active sensor 312. In some embodiments,the voltage follower 318 acts as a voltage buffer. The voltage follower318 can reduce the output loading of the voltage driver 308. In someembodiments, an active column includes more than one active sensor (thisconfiguration discussed below). In some embodiments, the bias voltage isdriven to more than one active column. The loading (e.g., capacitiveload) at the active bias voltage node increases as the size of thecolumn and/or the number of columns being driven increases. The voltagefollower 318 can maintain the size of the voltage driver 308 and adesirable active bias voltage response.

Although a voltage follower 318 is illustrated as being electricallycoupled between the voltage driver 308 and the active sensor 312, it isunderstood that the readout circuit 300 can include no voltage follower(e.g., the voltage driver 308 is electrically coupled to both secondterminals 310 b and 312 b, providing the active bias voltage) or morethan one voltage followers (e.g., multi-stage voltage buffers, parallelvoltage followers) without departing from the scope of this disclosure.

The symbol illustrating voltage follower 318 is used for illustrativepurposes only. It is apparent to a person of ordinary skill in the artthat different methods and circuits can be utilized to drive the voltageat the second terminal 312 b.

The first switch 330 is configured to selectively electrically couplethe reference sensor 310 to the first current source 304. The secondswitch 332 is configured to selectively electrically couple the activesensor 312 to the second current source 306. The third switch 334 isconfigured to selectively electrically couple the second referencesensor 320 to the first current source 304. The fourth switch isconfigured to selectively electrically couple the second active sensor322 to the second current source 306. The switches can be any suitablecomponents that can selectively electrically couple the circuitelements. For example, the switches are transistors. The switches canelectrically couple to a controller that closes the appropriate switchesbased on a readout scheme.

The circuit 300 can be configured for readout operation of a sensorarray including two or more rows. For example, the reference sensor 310and the active sensor 312 are associated with a first row of the sensorarray, and the second reference sensor 320 and the second active sensor322 are associated with a second row of the sensor array. During readoutof the array, the first row is selected for readout. When the first rowis selected for readout, the first switch 330 and the second switch 332are closed (conducting), coupling the reference sensor 310 to the firstcurrent source 304 and the active sensor 312 to the second currentsource 306, respectively. During this time, the third switch 334 and thefourth switch 336 are opened (not conducting), keeping the secondreference and active sensors electrically uncoupled from theirrespective current sources.

When the first row transitions to the second row for readout, the firstswitch 330 and the second switch 332 are open (not conducting),electrically uncoupling the reference sensor 310 to the first currentsource 304, and the active sensor 312 to the second current source 306,respectively. During this time, the third switch 334 and the fourthswitch 336 are closed (conducting), electrically coupling the secondreference sensor 320 to the first current source 304 and the secondactive sensor 322 to the second current source 306, respectively.

The mechanism described in this disclosure can cause a current to enteror exit the input 303 of the readout element 302. Accordingly, thereadout data of the currently selected active sensor can be subsequentlyprocessed. This process can be repeated for subsequent rows until theentire sensor array has been scanned or until the intended rows havebeen scanned.

Although row-to-row readout operation for an active sensor column isdescribed, it is understood that the operation is not limited to onecolumn. The operation can be performed in sequence or simultaneously forone or more columns of active sensors using one or more columns ofreference sensors.

It is also understood that although the switches are configured asillustrated, the switches can be suitably connected in different mannerswithout departing from the scope of this disclosure.

For example, in some embodiments, additional switches are electricallycoupled between the second terminals and the voltage follower or thebias voltage node. In some embodiments, a voltage follower may besupplemented with or replaced by additional switches electricallycoupled between the second terminals and voltage driver. When theadditional switches are coupled between the second terminals and thevoltage driver, the second terminals of the one or more active sensorsare not the same node in the circuit (with or without the voltagefollower). In these embodiments, a respective additional switch of aselected row is closed (conducting) while the other additional switchesof the non-selected rows are open (not conducting). In theseembodiments, the loading at the second terminal of the selected activesensor is reduced because the loading of the second terminals of thenon-selected active sensors are effectively removed. These embodimentsare especially beneficial in cases when selected active sensors frommultiple columns are simultaneously driven and the loading at the secondterminals of the selected active sensors from the multiple columnsaffects circuit response.

In some embodiments, the voltage driver 308 is an op amp. The output 309of the op amp 308 is electrically coupled to the second terminals of thereference sensors. The negative input of the op amp can electricallycouple to the first terminals of the reference sensors.

The selected reference sensor and the op amp can form an invertingamplifier. The amplifier can output a bias voltage to the secondterminals of the reference sensors. The output bias voltage can besubstantially the same as the bias voltage described elsewhere in thisdisclosure. The first terminals of the reference sensors electricallycouple to a negative input of the amplifier. Since the first currentsource 304 provides the first current to the first terminal, a voltagedrop across the negative input and the output is generated, and anappropriate bias voltage is driven by the amplifier based on the voltagedrop. A reference voltage can electrically couple to the positiveterminal of the op amp. For example, the reference voltage is a constantvoltage or a ground voltage.

A feedback element 338 is electrically coupled to the first and secondterminals of the reference sensor. In other words, the feedback elementcan provide an additional negative feedback path for the op amp 308. Inan embodiment, the feedback element is a capacitor. In anotherembodiment, the feedback element is a sensor substantially similar tothe reference sensor. The feedback element 338 can keep the feedbackloop of the voltage driver closed during row transitions, thuspreventing the voltage driver 308 from railing or saturating at an openloop situation. The capacitor can be sized appropriately such that thevoltage at output 309 will be the desired voltage at the start of thereadout of the next row. A person of ordinary skill in the art willrecognize that other suitable feedback elements can be used withoutdeparting from the scope of the disclosure.

In some embodiments, a first current buffer is coupled between thereference sensors and the first current source and a second currentbuffer is coupled between the active sensors and the input of thereadout element. In some embodiments, the current buffers shieldundesired effects at the input nodes (e.g., input of the readoutelement, negative terminal of voltage driver 308) from signals generatedby the reference and active sensors. For example, undesired current canbe injected into the input nodes and modulate the voltages at thesenodes. The current buffers prevent the signal currents from beingaffected by the undesired injected currents.

FIG. 4 illustrates a method 400 of sensor readout, in accordance with anembodiment. In some embodiments, method 400 is used in conjunction withmethod 200. In some embodiments, method 400 is performed using thereadout circuits described in this disclosure. Method 400 includesproviding a fourth current to a first terminal of a second referencesensor (step 402). For example, referring to FIG. 3, a first currentsource 304 can provide a fourth current to the first terminal 320 a ofthe second reference sensor 320. In some embodiments, the first currentand the fourth current have the same magnitude.

Method 400 can further include electrically uncoupling, from thereference sensor, the first current source providing the first current;and coupling, to the second reference sensor, the first current sourceproviding the fourth current. For example, the first switch 330 can open(not conduct), electrically uncoupling the reference sensor 310 to thefirst current source 304. The second switch 332 can close (conduct),coupling the second reference sensor 320 to the first current source304.

Method 400 includes generating, from the fourth current, a secondvoltage at a second terminal of the second reference sensor (step 404).For example, due to the fourth current and the impedance of the secondreference sensor 320, a voltage drop across the reference sensor 320 anda voltage at the second terminal 320 b are generated.

Method 400 includes providing a fifth current to a first terminal of asecond active sensor (step 406). For example, a second current source306 can provide a fifth current to the first terminal 322 a of thesecond active sensor 322. In some embodiments, the second current andthe fifth current can have the same value.

Method 400 can further include electrically uncoupling, from the activesensor, a second current source providing the second current; andcoupling, to the second active sensor, the second current sourceproviding the fifth current. For example, the third switch 334 can open(not conduct), electrically uncoupling the active sensor 312 to thesecond current source 306. The fourth switch 336 can close (conduct),coupling the second active sensor 322 to the second current source 306.

In some embodiments, the fourth current and the fifth current areconstant. In some embodiments, the first current source and the secondcurrent source are configured to output an equal magnitude of current ina same direction relative to the respective first terminals. Forexample, currents provided by the first current source 304 and secondcurrent source 306 can flow toward a reference potential 320. In someembodiments, the reference potential 320 is a reference voltage drivenby a voltage source. In another embodiment, the reference potential 320is a ground.

In some embodiments, the first current source and the second currentsource are selected from the group of an athermal voltage source andresistor, a high-impedance athermal transistor current source, and aWilson current mirror.

As an exemplary advantage, since constant and athermal current sourcescan be used to provide the currents of the readout circuit, the outputimpedance of the current sources can be high compared to the sensors.Thus, the efficiency of the circuit increases because more signalcurrent enters the readout element, instead of being divided by aresistive divider formed by reference and active sensors.

Method 400 includes driving, at the second voltage, a second terminal ofthe second active sensor (step 408). For example, the second terminal320 b of the second reference sensor 320 is electrically coupled to thesecond terminal 322 b of the active sensor 322, and the second voltageat this node is driven by voltage driver 308 at output 309. In someembodiments, the second voltage is a bias voltage for the second activesensor. For example, as described earlier, the bias voltage isreflective of common mode effects. By biasing the second active sensorat the voltage, these common mode effects can be compensated.

In some embodiments, the second voltage is driven by an op amp, and thefirst terminals of the reference sensors are electrically coupled to anegative input of the op amp. In some embodiments, the method furtherincludes feeding back from the second terminal of the reference sensorto the first terminal of the reference sensor using a feedback element.

In some examples, the fourth current causes a voltage drop across thesecond reference sensor; the second voltage is generated with anamplifier outputting to the second terminal of the second referencesensor; and the first terminal of the second reference sensor iselectrically coupled to a negative terminal of the amplifier.

Method 400 includes exposing the second active sensor to the sensorimage (step 410). For example, the second active sensor 322 is exposedto a sensor image. In some embodiments, exposing the second activesensor to the sensor image further includes changing an impedance of thesecond active sensor. For example, the second active sensor is a secondactive bolometer pixel. The second active bolometer pixel is exposed toa thermal scene. The impedance of the second active bolometer pixel canchange in response to exposure to the thermal scene. In someembodiments, exposing the second active sensor to the sensor imageincludes exposing the second active sensor to LWIR radiation.

Method 400 includes measuring a sixth current from the first terminal ofthe second active sensor to the input of a readout element (step 412).For example, a current entering or exiting input 303 of the readoutelement 302 is measured. In some embodiments, the sixth current iscaused by the mechanisms described in this disclosure.

Accordingly, the readout data of the currently selected active sensorcan be subsequently processed. This process can be repeated forsubsequent rows until the entire sensor array has been scanned or untilthe intended sensors have been scanned.

Although row-to-row readout operation for an active sensor column isdescribed, it is understood that the operation is not limited to onecolumn. The operation can be performed in sequence or simultaneously formore than one column of active sensors using more than one column ofreference sensors.

FIG. 5 illustrates a sensor readout circuit, in accordance with anembodiment. The readout circuit 500 includes components that aresubstantially similar to those described in FIGS. 1 and 3. Likecomponents in FIG. 5 are given like numerals as the correspondingcomponents in FIGS. 1 and 3. For the sake of brevity, those likecomponents are not described again with respect to FIG. 5. In someembodiments, components of circuit 500 are utilized in conjunction withthe readout circuits described elsewhere in the disclosure. Someembodiments include a method of manufacturing the readout circuit 500.

The circuit 500 further includes a stage 530 of a readout element thatcan be substantially similar to the readout elements described in thisdisclosure. In some embodiments, the stage of the readout element is aCTIA. In some embodiments, because of the symmetric constructionsbetween the reference sensor and active sensor branches, the voltage atthe input 503 can ideally equal the voltage of the positive terminal 534when the reference sensor 510 and the active sensor 512 have the sameimpedance (for example, when the sensors are both exposed to only theambient conditions). In some embodiments, the voltage at the positiveterminal 534 is a reference voltage. In some embodiments the voltage atthe positive terminal 534 is a ground voltage.

First current buffer 550 is coupled between the reference sensors andthe first current source 504. A second current buffer 552 is coupledbetween the active sensors and the input 503. In some embodiments, thecurrent buffers shield the effects of feedback at the input nodes (e.g.,input 503, negative terminal of voltage driver 508) from signalsgenerated by the reference and active sensors. For example, undesiredcurrent can be injected into the input nodes and modulate the voltagesat these nodes. The current buffers prevent the signal currents frombeing affected by the undesired injected currents.

Although a current buffer is illustrated each for the reference andactive branches, it is understood that the readout circuit can includeno current buffer (e.g., the current sources are electrically coupled tothe reference and active sensors) or more than one current buffer (e.g.,parallel current buffers, a current buffer for each branch) withoutdeparting from the scope of this disclosure.

The symbol illustrating current buffers 550 and 552 are used forillustrative purposes only. The symbol does not necessary imply thatcurrent buffering is performed using only drivers or amplifiers. It isapparent to a person of ordinary skill in the art that different methodsand circuits can be utilized to buffer the current between the sensorsand the current sources.

In some embodiments, the voltage at the output of the CTIA isproportional to an impedance difference between the reference sensor andthe active sensor during readout of a sensor. For example, the voltageat the output 536 of the CTIA is V, the voltage at the input 503 of theCTIA is V_(n), an effective capacitance at the input of the CTIA isC_(p), the open loop gain of the CTIA is A, the feedback capacitor ofthe CTIA can have a value of C_(CTIA), the impedance of the referencesensor 510 is Z_(ref), the impedance of the active sensor 512 isZ_(active), the bias voltage (i.e. the voltage at the second terminal512 b) of the active sensor 512 is V_(bias), the first current source504 can provide a current of I₁, the second current source 506 canprovide a current of I₂, and a voltage at reference potential 520 isV_(ref). The following equations can be used to calculate V:

$\begin{matrix}{V_{bias} = {I_{1} \times Z_{ref}}} & (1) \\{{\frac{\left( {V_{bias} - V_{n}} \right)}{Z_{active}} + {{sC}_{CTIA}\left( {V - V_{n}} \right)} + I_{2} - {{sC}_{p}V_{n}}} = 0} & (2) \\{{- {AV}_{n}} = V} & (3)\end{matrix}$

The polarities of the above variables are denoted only to solve thecircuit parameters. It is understood that the currents in the circuitcan be in any direction and the voltages in the circuit can be in anypolarity.

Combining the above equations, V can be isolated:

$\begin{matrix}{V = {I_{1} \times A \times \frac{\left( {\frac{Z_{ref}}{Z_{active}} - 1} \right)}{\frac{1}{Z_{active}} + \frac{I_{1}}{V_{ref}} + {s\left( {{C_{CTIA}\left( {A + 1} \right)} + C_{p}} \right)}}}} & (4)\end{matrix}$

The term ξ is a gain coefficient:

$\begin{matrix}{\xi = \frac{A}{\frac{1}{Z_{active}} + \frac{I_{1}}{V_{ref}} + {s\left( {{C_{CTIA}\left( {A + 1} \right)} + C_{p}} \right)}}} & (5)\end{matrix}$

The CTIA can be effectively an ideal integrator. In other words, theopen loop can be substantially large compared to the other variables inthe gain coefficient. In these cases, the variable A can effectivelyapproach infinity. In some examples, the open loop gain A is greaterthan 20. As such, the gain coefficient can be approximated:

$\begin{matrix}{\xi \approx \frac{1}{{sC}_{CTIA}}} & (6)\end{matrix}$

The effective integration capacitance of the CTIA is:

C _(e) =C _(CTIA)(A+1)+C _(p)   (7)

The effective integration resistance of the CTIA is:

$\begin{matrix}{R_{e} = {1\text{/}\left( {\frac{1}{Z_{active}} + \frac{I_{1}}{V_{ref}}} \right)}} & (8)\end{matrix}$

Assuming the CTIA is effectively an ideal integrator:

sR_(e)C_(e)>>1   (9)

Therefore, the CTIA has an integration time:

τ<<R_(e)C_(e)   (10)

Since the CTIA can be effectively an ideal integrator, the followingrelationship is derived for V:

$\begin{matrix}{V = {I_{1}{\xi \left( \frac{Z_{{ref} -}Z_{active}}{Z_{active}} \right)}}} & (11)\end{matrix}$

The reference sensor 510 and the active sensor 512 are identical instructure. In other words, the two sensors can have the same impedancewhen exposed to same conditions. Therefore, V can be further simplified:

ΔZ=Z _(ref) −Z _(active)   (12)

Where ΔZ is an impedance difference between the reference and activesensors. Since the terms I₁ and ξ are effectively constant during thereadout time, V is proportional to ΔZ, independent of any otherparameters.

V∝ΔZ/Z_(active)   (13)

Using the readout circuits described in this disclosure, a sensor imagecan be computed based only on an impedance difference between thereference and active sensors, independent of any other parameters. It isnoted that in addition to removing common mode effects from themeasurement, the disclosed constant current readout circuits and methodsalso yield the elegant result shown above.

From the above derivation, one can see that an exemplary advantage ofthe disclosed circuits and methods is that an output voltage of thereadout circuit can be substantially proportional to an impedancedifference of a reference sensor and an active sensor. As a consequenceof reading out in a constant current mode, some of the approximationsand cancellations in the above derivation can be made; the sensor imagecan be simply computed based on the proportional relationship. In someexamples, the simple proportional relationship allows the sensor imageto be more easily computed. As a result, less processing is required. Inother examples, the simple proportional relationship includes lessvarying and unknown elements (in the equations and on the circuit). As aresult, the sensor image is more accurate.

Although the term proportional is used to describe the relationshipbetween an output voltage and a sensor impedance difference, it isunderstood that the term “proportional” is not limited to an exactlinear relationship. Without departing from the scope of thisdisclosure, the term “proportional” can be used to describe anapproximate linear relationship between two quantities. The term“proportional” can also be used to describe a relationship between twoquantities that are different by a scaling factor.

Although the approximations above are made with the above assumptions,it is understood that the above variables can approach theirapproximated values when the assumptions are met in the circuit. Theapproximated value can be replaced in the equation, for the purpose ofcalculation, without any unexpected consequences.

The circuit 500 can have an additional load. The additional load can berepresented by capacitor 540 at input 532 of the stage 530. It isunderstood that the representative capacitor 540 is illustrative and notan actual capacitor or capacitive component electrically coupled toground. In some embodiments, the representative capacitor 540 is aneffective capacitance at the input 503 node, which is representative ofnon-ideal capacitances (e.g., sensor load capacitances, current sourcecapacitances, mismatches, wiring capacitances, parasitics) of thecircuit.

To remove the undesired additional load, the readout circuit can includea CDS circuit. In some embodiments, the CDS circuit is included as partof the CTIA. To remove the additional load, prior to reading out aselected sensor, the effect on the circuit caused by the additional loadis determined. For example, the effective charge caused by the effectiverepresentative additional load (e.g., capacitance 540) is sampled. Theeffect of the additional load is canceled prior to reading out theselected sensor.

In some embodiments, the additional non-idealities can include voltageand current non-idealities caused by elements of the readout circuit(not symbolically represented). In some embodiments, the CDS circuitremoves the additional non-idealities as well as the effects of thenon-ideal capacitances in the circuit.

A method of sensor readout can include converting a current inputted tothe readout element to a readout voltage of the readout element. Forexample, a CTIA can convert the current to a readout voltage of thereadout element. In some embodiments, a voltage at an output of the CTIAis proportional to an impedance difference between the reference sensorand the active sensor.

The readout circuit can further include a CDS circuit. The CDS circuitcan remove an offset in the readout circuit. In some embodiments, theCDS circuit is included as part of the CTIA.

In some embodiments, the method further includes: determining an offsetgenerated by the input of the readout element; and canceling the offsetprior to measuring the current to the input of the readout element.

In some embodiments, the circuit can include additional non-idealitiessuch as voltage and current non-idealities caused by elements of thereadout circuit (not symbolically represented). In some embodiments, theCDS circuit removes the additional non-idealities as well as the offsetin the circuit.

FIG. 6 illustrates a sensor readout circuit, in accordance with anembodiment. The readout circuit 600 can include components that aresubstantially similar to those described in FIGS. 1, 3, and 5. For thesake of brevity, those like components are not described again withrespect to FIG. 6. In some embodiments, components of circuit 600 areutilized in conjunction with the readout circuits described elsewhere inthe disclosure. Some embodiments include a method of manufacturing thereadout circuit 600.

In some embodiments, the readout circuit 600 includes a third referencesensor 630 and a third current source 608. The third reference sensor630 includes a first terminal 630 a and a second terminal 630 b that areelectrically coupled to a voltage driver (not shown). The third currentsource 608 is electrically coupled to the first terminal 630 a of thethird reference sensor 630, and is configured to output a seventhcurrent reflective of self-heating generated by the third referencesensor 630. The value of the second current provided by second currentsource 606 adjusts in accordance with the seventh current. In anexample, the second current source is substantially similar to thesecond current described in this disclosure, and the third referencesensor is a reference or blind bolometer pixel.

In an embodiment, the seventh current is provided by a controlledcurrent source. The controlled current source is controlled usingfeedback loop 616, which tracks the voltage at the first terminal 630 aof the third reference sensor 630, and hence, its self-heating. For acurrently selected row of sensors, the seventh current can change due tothe tracked self-heating of the third reference sensor 630, which isreflective of self-heating experienced by the active sensors. Thevoltage at the first terminal 630 a is held constant by the op-amp offeedback loop 616 in a negative feedback configuration. The outputvoltage of the op-amp updates to track the changes in current due toself-heating. The seventh current is mirrored to the second currentsource 606, which is electrically coupled to active sensor 612 tocompensate for self-heating of the selected active sensor.

A negative feedback amplifier is shown merely for exemplary purposes. Itis understood that other components and methods can track and controlthe currents 606 and 608 without departing from the scope of theinvention. Although a voltage driver is not shown to drive the biasvoltage, it is understood that a voltage driver can be electricallycoupled across the reference sensor to drive the bias voltage, asdescribed in this disclosure. Although one reference column and oneactive column are illustrated, it is understood that one or morereference columns can track self-heating and one or more active columnscan be compensated for self-heating.

FIG. 7 illustrates a method 700 of current adjustment, in accordancewith an embodiment. For example, a current provided to an activebolometer pixel is compensated for self-heating by tracking a blindbolometer pixel.

Method 700 includes: providing a seventh current to a first terminal ofa third reference sensor (step 702), the seventh current reflective ofself-heating generated by the third reference sensor; and adjusting avalue of the second current in accordance with the seventh current (step704). Method 700 can be achieved using readout circuit 600 or othersuitable self-heating compensating components. For example, the seventhcurrent is provided by third current source 608 and tracked by feedbackloop 616. The tracked current is mirrored to a current (e.g., currentprovided by second current source 606) provided to an active sensor.

Alternatively, in an embodiment, the voltage across the reference sensoris kept constant. In other words, the third reference sensor 630 is notexposed to ambient conditions and no voltage driver is electricallycoupled in parallel with the third reference sensor 630. In thisembodiment, the third reference sensor does not generate the biasvoltage and adjust the average bias current to track ambient changes.However, it is necessary to generate a bias voltage and adjust the biascurrent with respect to ambient conditions in order to maintain aconstant gain in the pixel response.

FIG. 8 illustrates a sensor bias circuit, in accordance with anembodiment. The readout circuit 800 can include components that aresubstantially similar to those described in FIGS. 1, 3, 5, and 6. Forthe sake of brevity, those like components are not described again withrespect to FIG. 8. In some embodiments, components of circuit 800 areutilized in conjunction with the readout circuits described elsewhere inthe disclosure. Some embodiments include a method of manufacturing thecircuit 800.

The sensor bias circuit 800 separately tracks the ambient condition andis independent of the described self-heating compensation components.Circuit 800 includes a reference sensor 840, a current source 810, and avoltage driver 808. For example, the reference sensor 802 is a thermallyshorted (blind or reference) bolometer pixel. In some embodiments, thecurrent source 810 and the voltage driver 808 is substantially the sameas the first current source and voltage driver described in thedisclosure. The bias voltage, which tracks the ambient conditions, isgenerated in a substantially the same manner as described in thisdisclosure. The bias voltage output 809 is electrically coupled to thebias voltage nodes of the readout circuits described in this disclosureto generate the active sensor bias currents.

By separating ambient condition tracking from the self-heatingcompensation, faster self-heating compensation can be achieved.Compensation response can be faster by adjusting current mirrors at eachrespective column, instead of using a global amplifier to drive arelatively large load at the bias voltage node. Since circuit sizing isless restricted by the load at the bias voltage node, a greater range ofpixel array sizes can be compatible with same periphery circuits. Inother words, the readout circuits can be more scalable. Although onevoltage driver and one bias circuit are illustrated in the figure, it isunderstood that one or more of the illustrated components can be used toform one or more sensor bias circuits.

In some embodiments, a voltage follower is electrically coupled betweenthe bias voltage node and second terminals of one or more activesensors, in a manner similar to the voltage followers described herein.In some embodiments, the voltage follower acts as a voltage buffer. Thevoltage follower can reduce the output loading of the voltage driver808. In some embodiments, an active column includes more than one activesensor. In some embodiments, the bias voltage is driven to more than oneactive column. The loading (e.g., capacitive load) at the active biasvoltage node increases as the size of the column and/or the number ofcolumns being driven increases. The voltage follower can maintain thesize of the voltage driver 808 and a desirable active bias voltageresponse.

FIG. 9 illustrates a sensor readout system, in accordance with anembodiment. Sensor readout system 900 includes sensor readout circuit902 and ADC 904. The sensor readout circuit 902 can be a readout circuitdescribed in this disclosure. The sensor readout circuit 902 canelectrically couple to ADC 904. Some embodiments include a method ofmanufacturing the circuit 900.

In some embodiments, the ADC can sample a current or charge from thefirst terminal of the active sensor to the input of the readout element,as described in this disclosure. In some embodiments, the ADC 904includes one or more Sigma-Delta ADCs. In some examples, a first stageof the ADC is a CTIA. In these examples, common components (e.g., CTIA)between the readout circuit and the ADC are advantageously shared,optimizing system area and power.

In some embodiments, a method of sensor readout includes sampling avoltage generated by the current or charge to the input of the readoutelement. For example, the readout element can include a Sigma-Delta ADC,and the voltage being sampled is generated by a CTIA. In someembodiments, a first current buffer is coupled between reference sensorsand a first current source. A second current buffer is coupled betweenactive sensors and an input of the Sigma-Delta ADC. In some embodiments,the current buffers shield the effects of feedback at input nodes (e.g.,the input of the Sigma-Delta ADC) from signals generated by referenceand active sensors. For example, undesired current can be injected intothe input nodes and modulate the voltages at these nodes. The currentbuffers reduce the undesired injected currents' effect on the signalcurrents.

The input current of the CTIA is a difference generated by an activebolometer pixel exposed to a thermal scene. In some examples, the CTIAis a first stage of the Sigma-Delta ADC.

Examples of the disclosure are directed toward MEMS sensor calibrationcircuits and methods that overcome the herein-identified drawbacks. Insome embodiments, the sensor calibration circuits include a calibrationsensor and a calibration readout element. In some embodiments, thesensor calibration circuits include a calibration current source. Insome embodiments, a shutter is included with the sensor calibrationcircuits.

In some embodiments, the methods include measuring a voltage of thecalibration sensor and computing a calibrated readout voltage based onthe measured calibration sensor voltage. In some embodiments, themethods include measuring a readout voltage of a readout elementelectrically coupled to the calibration current source and computing anoutput based on the readout voltage. In some embodiments, the methodsinclude measuring readout voltages when the shutter is closed and whenthe shutter is opened and computing a difference between the readoutvoltages.

As an exemplary advantage, the disclosed circuits and methods removenoise at reduced costs. The circuits presented herein efficiently andcompactly remove noises in the sensor array. Thus, the clarity ofmeasured sensor images can be improved.

FIG. 10 illustrates a sensor readout circuit 1000, in accordance with anembodiment. The sensor readout circuit 1000 includes readout elements1002 a and 1002 b, current sources 1004 a to 1004 c, a voltage driver1006, reference sensors 1008 a and 1008 b, active sensors 1010 a to 1010d, and switches 1016 a to 1016 h. Some embodiments include a method ofmanufacturing the readout circuit 1000.

In some embodiments, elements of the sensor readout circuit 1000correspond to elements of the sensor readout circuit 100. In someembodiments, readout element 1002 a corresponds to readout element 102,current source 1004 a corresponds to second current source 106, currentsource 1004 c corresponds to first current source 104, voltage driver1006 corresponds to voltage driver 108, reference sensor 1008 acorresponds to reference sensor 110, and active sensor 1010 acorresponds to active sensor 112.

The topology, as exemplified in FIG. 10, uses one voltage driver, whichprovides a bias voltage. Without adding circuitry to reduce undesiredcommon mode effects, the topology can reduce undesired common effectswithout increasing complexity, size, and cost, introducing moreparasitics and unknowns, consuming more power, and adding noise inducingcomponents.

In some embodiments, the reference sensors 1008 a and 1008 b areshielded from the sensor image and the active sensors 1010 a to 1010 dare exposed to the sensor image. The reference sensors can be exposed toan ambient condition common to the reference sensors and active sensors.For example, the reference sensor is a reference or blind bolometerpixel that is exposed to ambient temperatures, but is not exposed to athermal scene, and the active sensor is an active bolometer pixel, whichis exposed to both the ambient temperatures and the thermal scene.Generally, the sensors include two terminals and can have a variableimpedance value between the two terminals that depends on a sensorimage. Although block symbols are used to represent the sensors in thisdisclosure, it is understood that the described sensor properties andcomponents are exemplary.

In some embodiments, during sensor readout, each row is readoutsequentially. For example, at a first row readout time, row control 1014a is driven by a voltage that turns on the switches of the correspondingrow (e.g., switches 1016 a to 1016 d) while row control 1014 b is drivenby a voltage that turns off the switches of the corresponding row (e.g.,switches 1016 e to 1016 h). At a second row readout time, row control1014 b is driven by a voltage that turns on the switches of thecorresponding row (e.g., switches 1016 e to 1016 h) while row control1014 a is driven by a voltage that turns off the switches of thecorresponding row (e.g., switches 1016 a to 1016 d). When switches of acorresponding row are turned on, the active and reference sensors of thecorresponding row are electrically coupled to the bias voltage node 1012and corresponding current sources while sensors of the other rows areelectrically uncoupled from the bias voltage node 1012 and correspondingcurrent sources. It is understood that the described readout operationis exemplary.

In some embodiments, a selected reference sensor generates an adjustablebias voltage at the bias voltage node 1012 that tracks both ambientconditions and self-heating. In such examples, the readout circuitoperates in a constant current mode. Since no other branch iselectrically coupled between current source 1004 c and the selectedreference sensor, the current through the selected reference sensor isconstant.

Because the reference sensor is exposed to ambient conditions, theimpedance of the reference sensor stabilizes to a value that isreflective of the ambient conditions. Although the impedance of thesensor can be substantially fixed at a stable state, it is understoodthat the impedance of the reference sensor can vary according to theambient conditions. A voltage drop is generated across the referencesensor due to the current and the reference sensor impedance. Thevoltage at the bias voltage node 1012 is dictated by the voltage drop.

The voltage at the bias voltage node 1012 is driven by voltage driver1006. In an embodiment, voltage driver 1006 can act substantially as anideal voltage source. In other words, the voltage driver 1006 canprovide (or absorb) the necessary current to maintain the generatedvoltage at the bias voltage node 1012.

The symbol illustrating voltage driver 1006 is used for illustrativepurposes only. It is apparent to a person of ordinary skill in the artthat different methods and circuits can be utilized to drive the biasvoltage.

The bias voltage can be significant because it is reflective of commonmode effects, such as self-heating and ambient conditions, observed bythe reference sensor. As such, the selected active sensors are biased atthe bias voltage, and the common mode effects (which candisadvantageously reduce the dynamic range of subsequent stages) havebeen effectively compensated, because the bias voltage is reflective ofcommon mode conditions.

In some embodiments, an active sensor is substantially the same as areference sensor, and the current sources 1004 a-1004 c providesubstantially the same current.

In some examples, the readout elements can have a low input resistance,such as an input of a charge amplifier. In these examples, if thereference and active sensors are exposed to substantially the sameconditions, and the current sources provide substantially the samecurrents, no current would enter or exit the input of the readoutelements because the current traversing a respective selected activesensor would be the same as the current provided by a respective currentsource. An example of this condition is when the exposed sensor image isthe ambient condition itself.

Conversely, if the reference and active sensors are exposed to differentconditions (i.e., the exposed sensor image is different than the ambientconditions), then a current would enter or exit the input of the readoutelement because the impedance of a respective selected active sensor isdifferent from the impedance of a selected reference sensor, and thevalues of the provided currents remain the same after exposure. Thecurrent or total charge entering or exiting a respective readout elementis captured and measured.

Since common mode effects such as self-heating and ambient conditionshave been compensated by the bias voltage, the measured current orcharge is independent of these common mode components. As a result, thedynamic range of subsequent stages can be reduced. Since the measuredcurrent or charge is independent of undesired common mode components,these common mode components are not part of the measurement.

In some embodiments, the sensors are located on a glass substrate. Insome embodiments, the glass substrate includes a plurality of TFTs thatare used as switches for the sensor array (e.g., switches 1016 a-1016h). In some embodiments, non-sensor and non-switching elements off thearray (e.g., readout elements, voltage drivers, current sources) areimplemented on one or more chips away from the array.

Although FIG. 10 illustrates the sensor readout circuit as having atwo-by-two active sensor array, two reference sensors, and otherdescribed elements, it is understood that the disclosed sensor readoutcircuit is exemplary.

Although the measured current or charge is independent of common modeeffects, it may be susceptible to noise, which affects accuracy of thesensor image measurement and clarity of the sensor image. As an example,bolometer noise can be overwhelming and the thermal image can bedistorted due to noise. Noise in a bolometer array can includenon-patterned noises and patterned noises. Examples of non-patternednoises include 1/f noise, thermal noise and process dependent noises inthe pixels. Examples of patterned noises include row-to-row patterns andcolumn-to-column patterns.

FIG. 11 illustrates a sensor calibration circuit, in accordance with anembodiment. In an embodiment, the sensor calibration circuit 1100includes a calibration sensor 1102, a calibration readout element 1104,and a current source 1106. In some embodiments, the calibration readoutelement 1104 is substantially the same as one of readout elements 1002 aor 1002 b. In some embodiments, current source 1106 is substantially thesame as one of the current sources 1004 a, 1004 b, or 1004 c. Someembodiments include a method of manufacturing the circuit 1100.

The calibration sensor 1102 can include two terminals: a first terminalelectrically coupled to bias voltage node 1108 of sensor readout circuit1110 and a second terminal electrically coupled to the calibrationreadout element 1104 and current source 1106.

For example, the sensor readout circuit 1110 is substantially the sameas sensor readout circuit 1000. The bias voltage node 1108 can be sharedby active sensors of the sensor readout circuit 1110. In someembodiments, the calibration sensor is shielded from a sensor image thatis exposed to active sensor elements of the sensor readout circuit 1110.

As another example, the sensor readout circuit 1110 is a bolometerreadout circuit that measures a thermal image; the bolometer readoutcircuit includes a full array of bolometers and associated row andcolumn readout circuitries. In this example, the calibration sensorelectrically coupled to the bolometer readout circuit is a calibrationbolometer.

The sensor readout circuit 1000 is merely exemplary. It is understoodthat the sensor readout circuit 1110 can have different configurationsand components without departing from the scope of the disclosure.

In some embodiments, the impedance of calibration sensor 1102 issubstantially the same as the nominal impedance of an active sensor inthe calibration readout circuit 1110. In other words, the impedanceratio of the two sensors is one. In some embodiments, the impedanceratio is temperature independent. In some embodiments, the nominalimpedance of a sensor is the impedance of the sensor when the sensor isnot exposed to a sensor image. In some examples, the nominal impedanceof the active sensor is in the range of 10 kiloohms to 100 megaohms.

In some embodiments, the calibration sensor 1102 has a higher electricalcarrier count than an electrical carrier count of an active sensor(e.g., one of active sensors 1010 a to 1010 d) in the calibrationreadout circuit 1110. As an example, the calibration sensor has a higherelectrical carrier count than the electrical carrier count of the activesensor if the calibration sensor has a greater quantity of electricallyconductive material compared to the active sensor.

For example, the calibration sensor and the active sensor can have thesame impedance while the calibration sensor has a higher electricalcarrier count when the calibration sensor and the active sensor have thesame aspect ratio (e.g., both are squares), but the physical dimensionsof the calibration sensor are greater than those of the active sensor.In some embodiments, the calibration sensor is formed by more than onesensor; the sensors are connected in series and parallel to achievetotal impedance that is substantially the same as the active sensor.Since the calibration sensor has a higher electrical carrier count, thecalibration sensor has less 1/f noise compared to an active sensor.

In some embodiments, the calibration sensor and the active sensor arefabricated from materials having a same temperature coefficient ofresistance (TCR). In some embodiments, the calibration sensor and activesensors are created from one or more of amorphous silicon, vanadiumoxide, platinum, titanium, titanium oxide, tungsten oxide, andmolybdenum. In some examples, the calibration sensor and the activesensor are fabricated from a substantially same material.

In some embodiments, the calibration readout element is ananalog-to-digital converter (ADC). For example, the calibration readoutelement is an ADC that is substantially the same as an ADC electricallycoupled to a column of the active sensors.

In some embodiments, the calibration readout circuit 1100 is used tocalibrate for noise in the measurement of active sensors. Specifically,the calibration readout circuit 1100 is used to calibrate themeasurement for row-to-row pattern noises, which are noises that variesfrom one row to another.

In an exemplary sensor calibration, during readout time, the readoutvoltages of selected active sensors (e.g., sensors of a selected row)and the readout voltage of the calibration sensor are measured withrespective readout elements. As described, the readout voltages ofselected active sensors are generated by currents caused by the activesensor's exposure to a sensor image. In some embodiments, the readoutvoltage of the calibration sensor is created by a current traversingbetween the first and second terminals of the calibration sensor and ismeasured by the calibration readout element 1104. The current isgenerated from current source 1106.

After the readout voltages of the selected active sensors and thereadout voltage of the calibration sensor are measured, a calibratedreadout voltage can be computed for a j^(th) column while the i^(th) rowis being measured:

$\begin{matrix}{{A_{{cal},{ij}}(t)} = {{A_{sij}(t)} - {{A_{iM}(t)}\frac{Z_{M}}{Z_{sij}}}}} & (14)\end{matrix}$

The index (i, j) is associated with a specific active sensor of theselected sensors. For example, the sensors of the i^(th) row are beingselected for readout and each of the selected sensors corresponds to aj^(th) column. Although the index (i,j) is used to described anexemplary sensor, it is understood that the subscripts can be used todescribe any active sensor.

The quantity A_(cal,ij) is the calibrated readout voltage associatedwith sensor (i,j)'s exposure to a sensor image. The quantity A_(s,ij) isthe (non-calibrated) readout voltage of sensor (i, j) at a respectivereadout element of the j^(th) column. The quantity A_(iM) is the readoutvoltage of the calibration sensor when the bias voltage node is drivenwith a bias voltage associated with the reference sensor of i^(th) row(e.g., bias voltage associated with the common mode conditions of thei^(th) row). Z_(M) is the impedance of the calibration sensor, andZ_(sij) is the nominal impedance of the sensor (i,j) (e.g., whenshielded from the sensor image). In some embodiments, the two impedancesare substantially equal, so the factor (Z_(M)/Z_(sij)) is one.

By including simple parameters such as the calibration sensor'simpedance and its readout voltage, equation (14) removes undesiredrow-to-row pattern noises, such as noise in the bias voltage, from theactive sensor readout voltage. The inclusion of the calibration sensor,an associated readout element, and an associated current source may beimportant for deriving a measurement that is free of row-to-row patternnoises. Without these elements and equation (14), the bias voltage maynot be measured directly and the effects of bias voltage noise may notbe correctly captured, because, due to pixel self-heating, the biasvoltage is a time varying voltage during ADC conversion time. Sinceundesired bias voltage effect can be removed for each row, the need tocalibrate the skimming current sources can be eliminated, reducing thecomplexity of the readout circuit.

The exemplary sensor calibration shows how a selected row of sensorsduring readout can be calibrated with the calibration sensor, removingundesired row-to-row pattern noises, such as bias voltage noise. In someembodiments, the described method is repeated for other selected rowsduring readout. For example, after a selected first row of sensors iscalibrated with the calibration sensor electrically coupled to areference sensor of the first row (e.g., reference sensor 1008 a) (i.e.,bias voltage associated with the common mode conditions of the firstrow), the first row of sensors is deselected and a second row of sensorsis selected for readout. The selected second row of sensors iscalibrated with the calibration sensor, which is now electricallycoupled to a reference sensor of the second row (e.g., reference sensor1008 b) (i.e., bias voltage associated with the common mode conditionsof the second row). Row-to-row pattern noises, such as bias voltagenoise associated with the second row, can be removed in a similar manneras described.

While more than one row of sensors can be calibrated during readout andevery row of sensors can be calibrated during readout, calibration maynot be necessary for an entire sensor array. For example, a subset ofall rows can be calibrated with the calibration sensor. The frequency ofcalibration with the calibration sensor may depend on noisecharacteristics of the system or environment.

FIG. 12 illustrates a method 1200 of sensor calibration, in accordancewith an embodiment. Method 1200 includes electrically coupling a firstterminal of a calibration sensor to a bias voltage node shared by aplurality of active sensors (step 1202). For example, referring to FIG.11, the calibration sensor 1102 is electrically coupled to bias voltagenode 1108.

Method 1200 includes electrically coupling a second terminal of thecalibration sensor to a calibration readout element (step 1204). Forexample, referring to FIG. 11, the calibration sensor 1102 iselectrically coupled to the calibration readout element 1104.

Method 1200 includes exposing the plurality of active sensors to asensor image (step 1206). For example, referring to FIG. 11, a pluralityof active sensors in sensor readout circuit 1110 is exposed to a sensorimage.

Method 1200 includes shielding the calibration sensor from the sensorimage (step 1208). For example, referring to FIG. 11, the calibrationsensor 1102 is shielded from the sensor image.

Method 1200 includes measuring, with a readout element, a readoutvoltage of an active sensor of the plurality of active sensors (step1210). For example, referring to FIG. 11, A_(sij), the non-calibratedreadout voltage of sensor (i, j), is measured.

Method 1200 includes measuring, with the calibration readout element, areadout voltage of the calibration sensor (step 1212). For example,referring to FIG. 11, the readout voltage of calibration readout element1104 is measured.

Method 1200 includes computing a calibrated voltage (step 1214). Forexample, the calibrated voltage is computed as a difference between (1)the readout voltage of the active sensor and (2) the readout voltage ofthe calibration sensor weighted by a ratio between an impedance of thecalibration sensor and an impedance of the active sensor. For example,referring to FIG. 11 and equation (14), the quantity A_(cal,ij) iscomputed based on the measured quantities and sensor impedances.

In some embodiments, the impedance of the calibration sensor is the sameas the impedance of the active sensor, and an electrical carrier countof the calibration sensor is greater than an electrical carrier count ofthe active sensor. In some embodiments, the ratio is one. In someembodiments, the ratio is temperature independent.

In some embodiments, the calibration sensor and the active sensor aremade from materials having a same TCR.

In some embodiments, the method 1200 includes electrically coupling acurrent source of a plurality of current sources to the second terminalof the calibration sensor and to the calibration readout element;electrically coupling a column of a plurality of columns of activesensors to the readout element, the column of active sensors includingthe active sensor; and electrically coupling a second current source ofthe plurality of current sources to the readout element. For example,referring to FIGS. 10 and 11, current source 1106 is electricallycoupled to calibration readout element 1104, a column of active sensorsis electrically coupled to a readout element in the sensor readoutcircuit 1000, and a current source is electrically coupled to a readoutelement in the sensor readout circuit 1000.

In some embodiments, the method 1200 includes closing a shutter;measuring, with the readout element, a first readout voltagecorresponding to the closed shutter; and measuring, with the calibrationreadout element, a second readout voltage corresponding to the closedshutter; and after computing the calibrated voltage, computing a seconddifference between (1) the calibrated voltage and a difference between(2a) the first readout voltage and (2b) the second readout voltageweighted by the ratio, where the second difference is a shuttercalibrated voltage. Examples of these embodiments will be describedbelow with reference to equations (22) and (23).

In some embodiments, the calibration readout element includes an ADC. Insome embodiments, the readout element includes an ADC.

In some embodiments, the plurality of active sensors and the calibrationsensor are bolometers, and the sensor image is a thermal image.

FIG. 13 illustrates a sensor calibration circuit, in accordance with anembodiment. In an embodiment, the sensor calibration circuit 1300includes calibration current sources 1302 a and 1302 b and switches 1304a to 1304 f. Some embodiments include a method of manufacturing thecircuit 1100.

The sensor calibration circuit 1300 is electrically coupled to sensorreadout circuit 1310, which includes sections 1310 a and 1310 b. In someexamples, the sensor readout circuit 1310 is sensor readout circuit1000.

The switches 1304 c and 1304 d are associated with a first column 1312 aof active sensors. The switches 1304 e and 1304 f are associated with asecond column 1312 b of active sensors, different from the first column.The switches 1304 c and 1304 e can electrically couple or electricallyuncouple the calibration current source and a readout element of thecolumn. The switches 1304 d and 1304 f can electrically couple orelectrically uncouple active sensors of their respective column and areadout element of the column (e.g., readout elements 1002 a and 1002b). In some embodiments, the readout element is an ADC.

It is understood that the sensor readout circuit 1000 is merelyexemplary and any number of columns and any number of switches canelectrically couple or uncouple the calibration current source or activesensors and the active sensor columns. Although the columns of sensorsare symbolically included in a box that represents sensor readoutcircuit 1310 and the calibration circuit 1310 is symbolicallyillustrated outside the representative box, it is understood that theterm “column” is not necessarily arranged in a straight line. In someembodiments, a column of sensors is associated with one readout element.A column of sensors can come in any shape or form without departing fromthe scope of the disclosure.

The switches 1304 a and 1304 b can electrically couple the currentsources 1302 a and 1304 b, respectively, and a column that is undercalibration. In an embodiment, the switches 1302 a and 1304 belectrically couple one of the current sources and the sensor columns ata time.

Although transistor symbols are used to visually represent the switches,it is understood that other implementations of switching can existwithout departing from the scope of the disclosure.

Although FIG. 13 illustrates the sensor calibration circuit 1300 ashaving two current sources and two associated switches, it is understoodthat any number of calibration current sources and associated switchescan exist without departing from the scope of the disclosure. In anotherembodiment, the sensor calibration circuit 1300 has one calibrationcurrent source and one associated switch.

In an exemplary sensor calibration, a first column is in calibrationmode; a readout element of the first column is electrically coupled to afirst calibration current source of the calibration current sources.

For example, the column 1312 a is in calibration while a first row ofsensors is being readout. Switch 1304 c is closed (conducting) andswitch 1304 d is opened (non-conducting). A first calibration current ofthe calibration current sources (e.g., calibration current source 1302 athrough switch 1304 a or calibration current source 1302 b throughswitch 1304 b) is electrically coupled to the readout element of thefirst column (e.g., readout element 1002 a).

While the first column is in calibration, columns that are not incalibration are in the readout mode. The readout elements of the columnsin the readout mode are electrically coupled to selected active sensorsof the respective columns.

For example, the column 1312 b is in readout mode while the first row ofsensors is being readout. Switch 1304 e is opened (non-conducting) andswitch 1304 f is closed (conducting). The currently selected activesensor (e.g., active sensor 1010 b) is electrically coupled to thecolumn's readout element (e.g., readout element 1002 b).

The readout voltages of the columns (i.e., the columns in readout modeand the column in calibration) are measured. The readout voltage causedby the first calibration current (e.g., readout voltage of the columnelectrically coupled to calibration current source 1302 a or calibrationcurrent source 1302 b, readout voltage of readout element 1002 a) can bedenoted as A_(1j)(t).

During the readout of the first row, at a second time, the first columnmay still be in calibration, and the other columns may still be in thereadout mode. At this time, the readout element of the first column iselectrically uncoupled from the first calibration current source andelectrically coupled to a second current source of the calibrationcurrent sources.

For example, at this second time, the column 1312 a is in calibrationmode while a first row of sensors is being readout. Switch 1304 c isclosed (conducting), and switch 1304 d is opened (non-conducting). Asecond calibration current source of the calibration current sources(e.g., calibration current source 1302 b through switch 1304 b orcalibration current source 1302 a through switch 1304 a) is electricallycoupled to the readout element of the first column (e.g., readoutelement 1002 a). The readout voltage caused by the second calibrationcurrent (e.g., from calibration current source 1302 a or calibrationcurrent source 1302 b), can be denoted as A_(2j)(t). In someembodiments, since the values of the first and second calibrationcurrents are known, the values A_(1j)(t) and A_(2j)(t) capture a driftof the respective readout element (e.g., readout element 1002 a).

With the measurement of A_(1j)(t) and A_(2j)(t), readout voltage of anyactive sensor in the calibrated column can be calibrated. The readoutvoltage of the active sensor can be denoted as A_(sij)(t), where anactive sensor associated with the i^(th) row and j^(th) column is beingmeasured.

In this particular example, during this particular time, since theselected active sensor of the calibrated column (e.g., active sensor1010 a) is not electrically coupled to the readout element, the currentreadout voltage of this sensor is not measured (e.g., readout voltageassociated with active sensor 1010 a). In some embodiments, a previousvalue of A_(sij) or an interpolated value based on adjacent sensors isused for A_(sij)(t).

In other embodiments, a current readout voltage A_(sij)(t) is measuredeven when a column is being calibrated; the calibration current sourcesare electrically uncoupled from the readout element and the activesensor is electrically coupled to the readout element, where the currentreadout voltage of the active sensor is measured.

With the measurements of the terms A_(sij)(t), A_(1j)(t), and A_(2j)(t),an output D_(sij)(t) can be computed for a column j^(th) while thei^(th) row is being measured:

$\begin{matrix}{{D_{sij}(t)} = \frac{{A_{sij}(t)} - {\left( {{A_{1j}(t)} + {A_{2j}(t)}} \right)\text{/}2}}{{A_{2j}(t)} - {A_{1j}(t)}}} & (15)\end{matrix}$

Where the quantity D_(sij)(t) is a term that is linear to the currentinto the readout element of a column j (e.g., the current caused by thesensor image, as described). Since equation (15) accounts for the driftin the readout element, the term is independent of effects such as ADCgain, offset, skimming currents, and any time-varying quantities exceptI(t), which is the current into a respective readout element. In someembodiments, the quantity D_(sij)(t) is a stabilized digital output,which is a stabilized version of an output of a readout element (e.g.,an ADC).

$\begin{matrix}{{{D(t)} = {\frac{{A_{1}(t)} - {\left( {{A_{1}(t)} + {A_{2}(t)}} \right)\text{/}2}}{{A_{2}(t)} - {A_{1}(t)}} = \frac{{I(t)} - \overset{\_}{I}}{\Delta \; I}}}{{Where}\text{:}}} & (16) \\{\overset{\_}{I} = \frac{I_{1} + I_{2}}{2}} & (17) \\{{\Delta \; I} = {I_{2} - I_{1}}} & (18)\end{matrix}$

I₁ is the first calibration current. I₂ is the second calibrationcurrent.

In some embodiments, since the column has been calibrated, in subsequentrow readouts (e.g., selection of the second row for readout), thequantities A_(1j)(t) and A_(2j)(t) are used to compute subsequentoutputs corresponding to subsequent rows of the column that are beingread out, until the column is calibrated again (i.e., A_(1j)(t) andA_(2j)(t) are updated).

For example, a second row (e.g., active sensors 1010 c-1010 d) is beingselected for readout and the first column is no longer being calibrated(since the quantities A_(1j)(t) and A_(2j)(t) corresponding to the firstcolumn have been measured). In this example, the column 1312 a is inreadout mode while the second row of sensors is being readout. Switch1304 c is opened (non-conducting), and switch 1304 d is closed(conducting). The currently selected active sensor (e.g., active sensor1010 c) is electrically coupled to the column's readout element (e.g.,readout element 1002 a). As such, during the second row readout time,the term A_(sij)(t) is the readout voltage of the active sensor 1010 c.With the measurements of the terms A_(sij)(t), A_(1j)(t), and A_(2j)(t),an output D_(sij)(t) can be computed for the active sensor of the firstcolumn and second row. Generally, after the quantities A_(1j)(t) andA_(2j)(t) have been measured, the output D_(sij)(t) can be computed forany active sensor of the column using equation (15) when a respectivesensor is being readout.

By including the calibration current sources and measuring readoutvoltages of the columns, an output D_(sij)(t) is computed. The quantityD_(sij)(t) can be linearly proportional to the active sensor current(i.e., current caused by the sensor image), free of any othertime-varying or column-dependent noise parameter. Equation (15) removesundesired column-to-column pattern noises, such as mismatches betweenskimming currents, ADC gains, and ADC offsets. In addition, theseundesired noises can be removed while the active sensors are beingreadout. In other words, the configuration described with respect toFIG. 4 can allow calibration and readout to be performed in parallel.Therefore, under this configuration, dedicated calibration time (i.e.,no readout while calibration is being performed) may not be needed andmore rows can be readout during a given amount of time, improvingperformance and accuracy of the readout circuit.

The inclusion of the calibration currents (e.g., calibration currentsources 1302 a and 1302 b) and associated switches (e.g., switches 1304a, 1304 b, 1304 c, and 1304 e) can be used for deriving a quantity(e.g., the output) that can be free of column-to-column pattern noiseswhile readout is being concurrently performed. Without these elementsand equation (15), column-dependent noises, such as skimming currentmismatches, ADC gain mismatches, and ADC offset mismatches, may bereflected on the sensor image, distorting the clarity of the sensorimage or dedicated calibration time would be needed to remove thesenoises, degrading the performance of the readout circuit.

In some embodiments, one calibration current source (e.g., calibrationcurrent source 1302 a or 1302 b) is used for calibration, instead of twocalibration current sources. In these embodiments, only the stepsassociated with the first calibration current source are performed(i.e., with one calibration current source, the steps associated withthe second time of a row readout time are not performed). For example,when a column is in calibration with one calibration current source, areadout element of the column is electrically coupled to only one of thecalibration current sources.

For example, the column 1312 a is in calibration mode while a first rowof sensors is being readout. Switch 1304 c is closed (conducting), andswitch 1304 d is opened (non-conducting). A first calibration current ofthe calibration current sources (e.g., calibration current source 1302 athrough switch 1304 a or calibration current source 1302 b throughswitch 1304 b) is electrically coupled to the readout element of thefirst column (e.g., readout element 1002 a).

While the column is in calibration, columns that are not in calibrationmode are in the readout mode. The readout elements of the columns in thereadout mode are electrically coupled to selected active sensors of therespective columns.

For example, the column 1312 b is in readout mode while the first row ofsensors is being readout. Switch 1304 e is opened (non-conducting), andswitch 1304 f is closed (conducting). The currently selected activesensor (e.g., active sensor 1010 b or 1000 d) is electrically coupled tothe column's readout element (e.g., readout element 1002 b).

During this time, the readout voltages of the columns (i.e., the columnsin readout mode and the column in calibration) are measured. The readoutvoltage caused by the calibration current (e.g., readout voltage of thecolumn electrically coupled to calibration current source 1302 a orcalibration current source 1302 b, readout voltage of readout element1002 a) can be denoted as A_(j)(t). A_(sij)(t) can be measured in thesame manner as described (i.e., interpolated with adjacent sensors orestimated with a previous measurement when the respective column is incalibration; active sensor readout voltage is measured when therespective column is in readout mode).

Using one calibration current, with the measurements of the termsA_(sij)(t) and A_(j)(t), an output D_(sij)(t) can be computed for acolumn j^(th) while the i^(th) row is being measured:

D _(sij)(t)=A _(sij)(t)−A _(j)(t)   (19)

The method associated with equation (19) removes the effect of ADCmismatches in a simpler manner compared to the method associated withequation (15) (e.g., less measurements, less required elements, simplercomputation).

The above calibration methods with the calibration current(s) can beperformed throughout the sensor readout circuit during readout. Theexemplary methods can be repeated by putting different columns incalibration.

For example, during readout time, a first row is selected for readout. Afirst column is in the calibration mode, as described, while theremaining columns are in the readout mode; the quantities A_(1j)(t) andA_(2j)(t) associated with the first column are measured and outputs D(t)associated with the active sensors of the column can be computed usingthe method as described.

After the readout of the first row is complete, a second row is selectedfor readout. A second column is in calibration, which is similar to howthe first column is calibrated, as described. The remaining columns(e.g., columns other than the second column) are in the readout mode,which is similar to that of the method as described.

During the calibration of the second column, the quantities A_(1j)(t)and A_(2j)(t) associated with the second column are measured (e.g.,readout voltages caused by the first calibration current and the secondcalibration current on the readout element 1002 b) and outputs D(t)associated with the active sensors of the second column can be computedusing the method as described (i.e., interpolated with adjacent sensorsor estimated with a previous measurement when the respective column isin calibration; active sensor readout voltage is measured when therespective column is in readout mode).

In some embodiments, generally, after a column is calibrated (i.e., thequantities A_(1j)(t) and A_(2j)(t) associated with the column aremeasured), outputs D(t) associated with the active sensors of the columncan be computed using the method as described (i.e., a respective activesensor readout voltage (e.g., A_(sij)(t)) is measured; since A_(1j)(t)and A_(2j)(t) are known after calibration, the terms can yieldcorresponding outputs D_(sij)(t)).

In this example, different columns are calibrated during subsequent rowreadouts. However, it is understood that same columns can be repeatedlycalibrated. It is also understood that calibration may not be necessaryduring readout of every row. It is understood that the frequency ofcalibration mode activation may depend on system requirements or thenoise environment without departing from the scope of the disclosure.Although one column is being calibrated during each row readout time, itis understood that more than one column may be calibrated withoutdeparting from the scope of the disclosure. For example, the quantitiesA_(1j)(t) and A_(2j)(t) may be measured for multiple columns during arow readout time and the multiple columns are calibrated during one rowreadout time.

In some embodiments, the calibration mode is enabled periodically. Insome embodiments, for a particular column, the calibration period isdefined by a time between a first calibration of the particular columnand subsequent calibration of the particular column (i.e., time betweenupdates of A_(1j) and/or A_(2j) for the particular column). In someembodiments, the calibration period is determined by a drift of thereadout element. For example, if the readout element drifts quickly, thecalibration period is short. In another example, if the readout elementdrifts slowly, the calibration period is long.

In some embodiments, during the successive calibrations of theparticular column, the column is calibrated when different rows arebeing readout. For example, during a first calibration time, a firstcolumn is calibrated when a first row is being readout; during a secondcalibration time, the first column is calibrated when a second row isbeing readout. In this manner, a same active sensor's actual readoutvoltage would not be always left out (e.g., would not always beingestimated with a previous value or an interpolated value). For example,if the first column is always being calibrated during the first rowreadout time, then the current readout voltage of the active sensor ofthe first column and first row would always be interpolated withadjacent active sensors or estimated with previous readout voltages.

In some embodiments, calibration mode is enabled every one second. Insome embodiments, calibration mode frequency is determined by noisecharacteristics of the sensor readout circuit. In some embodiments, thecolumn selected for calibration mode is based on noise characteristicsof the sensor readout circuit.

FIG. 14 illustrates a method 1400 of sensor calibration, in accordancewith an embodiment. Method 1400 includes electrically coupling a readoutelement to an active sensor (step 1402). For example, referring to FIG.13, a readout element of sensor readout circuit 410 is electricallycoupled to an active sensor of sensor readout circuit 1310.

Method 1400 includes measuring, with the readout element, a firstreadout voltage of the active sensor (step 1404). For example, referringto FIG. 13 and equations (15) and (19), a first readout voltage (e.g.,A_(sij)(t)) is measured with the readout element.

Method 1400 includes electrically uncoupling the readout element fromthe active sensor (step 1406). For example, referring to FIG. 13, thereadout element of sensor readout circuit 1310 is electrically uncoupledfrom the active sensor of sensor readout circuit 1310.

Method 1400 includes electrically coupling a calibration current to thereadout element (step 1408). For example, referring to FIG. 13,calibration current 1302 a or calibration current 1302 b is electricallycoupled to the active sensor of sensor readout circuit 1310.

Method 1400 includes measuring, with the readout element, a secondreadout voltage caused by the calibration current (step 1410). Forexample, referring to FIG. 13 and equations (15) and (19), a secondreadout voltage (e.g., A_(j)(t), A_(1j)(t) is measured with the readoutelement.

Method 1400 includes computing an output based on (1) the first readoutvoltage and (2) the second readout voltage (step 1412). In someexamples, the output is proportional to a readout current of the activesensor. For example, referring to equations (15) and (19), the outputD(t) is computed based on the first and second readout voltages.

In some embodiments, the method 1400 includes electrically coupling arespective active sensor of a plurality of active sensors to arespective readout element of a plurality of readout elements;measuring, with the respective readout element, a first readout voltageof the respective active sensor; electrically uncoupling the respectivereadout element from the respective active sensor; electrically couplingthe calibration current to the respective readout element; measuring,with the respective readout element, a second readout voltage caused bythe calibration current on the respective readout element; and computingan output proportional to a readout current of the respective activesensor based on (1) the first readout voltage of the respective activesensor and (2) the second readout voltage caused by the calibrationcurrent. For example, referring to FIG. 4, a column 1312 b is calibrated(i.e., A_(j)(t) associated with column 1312 b is measured, the drift ofthe column is measured) after column 1312 a is calibrated (i.e.,A_(j)(t) associated with column 1312 a is measured, the drift of thecolumn is measured).

In some embodiments, the method 1400 includes, after computing the firstoutput: electrically uncoupling the calibration current source from thereadout element; electrically coupling the readout element to a secondactive sensor, the second active sensor belonging to a same column asthe first active sensor; measuring, with the readout element, a thirdreadout voltage of the second active sensor; and computing a secondoutput proportional to a readout current of the second active sensorbased on (1) the third readout voltage and (2) the second readoutvoltage caused by the calibration current. For example, referring toFIG. 13, after output D(t) is computed for a first row on column 1312 a(i.e., the drift of the column is measured), an output D(t) associatedwith a second row on column 1312 a can be computed based on A_(j)(t) andreadout voltage of the active sensor on the second row.

In some embodiments, a time between successive measurements of thesecond readout voltage on the same column caused by the calibrationcurrent is a calibration period. In some embodiments, the calibrationperiod is one second. In some embodiments, the calibration period isbased on a drift of the readout element.

In some embodiments, different rows are being readout during thesuccessive measurements of the second readout voltage. For example,referring to FIG. 13, the quantity A_(j)(t) is measured at a first timefor column 1312 a when a first row is readout. At a subsequent time, thequantity A_(j)(t) is measured for column 1312 a when a second row,different from the first row, is readout.

In some embodiments, the method 1400 includes electrically uncouplingthe readout element from the first calibration current source;electrically coupling the readout element to a second calibrationcurrent source; and measuring, with the readout element, a third readoutvoltage caused by the second calibration current on the readout element,where the output is further based on the third readout voltage caused bythe second calibration current. For example, referring to FIG. 13 andequation (15), both calibration currents 1302 a and 1302 b are used tomeasure the quantities A_(1j)(t) and A_(2j)(t), and the output D(t) isbased on the quantities A_(1j)(t) and A_(2j)(t).

In some embodiments, the readout element includes an ADC.

In some embodiments, the method 1400 includes closing a shutter;computing the output corresponding to a closed shutter; and computing adifference proportional to an impedance difference of the active sensorcaused by a sensor image between (1) the output corresponding to anopened shutter and (2) the output corresponding to the closed shutter.Examples of these embodiments will be described below with reference toequations (22) and (23).

In some embodiments, the first active sensor is a bolometer exposed to athermal scene.

FIG. 15 illustrates a sensor calibration circuit, in accordance with anembodiment. The calibration circuit 1500 includes calibration sensorcircuit 1520 and calibration current circuit 1530. In some embodiments,the calibration circuit 1500 is a combined circuit that performs thecalibration methods as described. For example, calibration sensorcircuit 1520 can be similar to sensor calibration circuit 1100.Calibration current circuit 1530 can be similar to calibration currentcircuit 1300. The calibration circuit 1500 is electrically coupled tosensor readout circuit 1510. In some embodiments, the sensor readoutcircuit 1510 can be substantially similar to sensor readout circuit1000. It is understood that readout circuit 1510 can include a sensorarray of any size and corresponding readout circuitries withoutdeparting from the scope of the disclosure. Some embodiments include amethod of manufacturing the circuit 1500.

FIG. 16 illustrates a sensor calibration circuit, in accordance with anembodiment. For example, sensor calibration circuit 1600 is anembodiment of the sensor calibration circuit 1500. The sensorcalibration circuit 1600 includes calibration sensor circuit 1620 andcalibration current circuit 1630. The sensor calibration 1600 iselectrically coupled to sensor readout circuit 1610, which includessection 1610 a and 1610 b, as shown. It is understood that FIG. 16 isillustrative and its elements are symbolic. Other configurations of thesensor calibration circuit 1600 and connections between the sensorcalibration circuit and the sensor readout circuit 1610 can existwithout departing from the scope of the disclosure. Some embodimentsinclude a method of manufacturing the circuit 1600.

Although some sections of the circuits are illustrated with dashedboxes, it is understood that the dashed boxes are merely added forclarity and are not meant to be limiting.

In some embodiments, the combined calibration circuit (e.g., sensorcalibration circuit 1600) concurrently performs the calibration methodsassociated with FIGS. 11 and 13. For example, while a selected row ofsensors is being readout, a calibration sensor is used to calibrate thebias voltage associated with the selected row and the calibrationcurrents are used to calibrate the ADCs and skimming currents forparticular column(s). The combined calibration using the calibrationsensor and calibration currents includes substantially the same steps aseach of the individual methods as described. For the sake of brevity,the similar steps are not repeated here.

Using substantially the same method as described, an output associatedwith the calibration sensor is computed:

$\begin{matrix}{{D_{iM}(t)} = \frac{{A_{iM}(t)} - {\left( {{A_{M\; 1}(t)} + {A_{M\; 2}(t)}} \right)\text{/}2}}{{A_{M\; 2}(t)} - {A_{M\; 1}(t)}}} & (20)\end{matrix}$

-   -   Where the terms A_(M1)(t) is the readout voltage of the        calibration readout element when the first calibration current        is electrically coupled to the calibration readout element and        A_(M2)(t) is the readout voltage of the calibration readout        element when the second calibration current is electrically        coupled to the calibration readout element. In some embodiments,        since calibrating the calibration readout element precludes the        bias voltage from being calibrated (i.e., the calibration sensor        is electrically uncoupled from the calibration readout element),        a blank row readout time (e.g., one row readout time delay) can        be used to make the A_(M1)(t) and A_(M2)(t) measurements by        electrically coupling and uncoupling the respective calibration        current to the calibration readout element. In some embodiments,        the bias voltage is calibrated before the calibration sensor is        electrically uncoupled from the calibration readout element to        make the A_(M1)(t) and A_(M2)(t) measurements.

In the combined sensor calibration method, equations (15) and (20) canbe combined into equation (14). For the sake of brevity, the termsassociated with these equations are not described again. In anotherembodiment, when one calibration current is used, equation (20) isreduced to be substantially similar to equation (19) in a similar mannerthat equation (20) is derived from equation (15).

$\begin{matrix}{P_{ij} = {{\left( {D_{sij}(t)} \right) - {\left( {D_{iM}(t)} \right)\frac{Z_{M}}{Z_{sij}}}} \cong {{- \frac{1}{\Delta \; I}}\frac{V_{baisi}}{R_{ij}}\frac{\Delta \; R_{ij}}{R_{ij}}}}} & (21)\end{matrix}$

Since the term P_(ij) is computed by combining the calibration sensorand calibration current methods, the term P_(ij) is free of bothrow-to-row and column-to-column pattern noises. The sensor calibrationcircuit 1600 includes the benefits associated with sensor calibrationcircuit 1100 and sensor calibration circuit 1300, as described.

In some embodiments, a shutter can be added to supplement the describedsensor readout circuits (e.g., sensor readout circuit 1000) to removenoises associated with sensor variation and 1/f noise.

For example, at t=0, the shutter closes, temporarily shielding theactive sensors from the sensor image. Readout voltages of each columnare measured when the shutter is closed. The readout voltages measuredwhile the shutter is closed can be denoted as As_(ij()0). At a differenttime (e.g., at time t), the shutter opens, exposing the active sensorsto the sensor image. Readout voltages of each column are measured whenthe shutter is opened. The readout voltages measured while the shutteris opened can be denoted as As_(ij)(t).

With As_(ij)(t) and As_(ij)(0), the impedance change caused by thesensor image (e.g., ΔZ_(ij)) can be computed:

$\begin{matrix}{{{A_{sij}(t)} - {A_{sij}(0)}} \cong {{- g}\frac{V_{biasi}}{Z_{ij}}\frac{\Delta \; Z_{ij}}{Z_{ij}(t)}}} & (22)\end{matrix}$

-   -   Where g is the gain of the readout element associated with        measurements, V_(biasi) is the bias voltage associated with the        reference sensor of the i^(th) row (i.e., the currently selected        row), and Z_(i)(t) is the absolute impedance of sensor (i,j)        when As_(ij)(t) is measured (e.g., when the shutter is opened).

Shutter calibration can remove noise patterns due to sensor variationand reset 1/f noise associated with the sensors and circuit elements. Insome embodiments, sensor requirements determine the frequency of theshutter. For example, a sensor requirement is bolometer pixel noiseequivalent temperature difference (NETD).

In some embodiments, the described shutter calibration method iscombined with the sensor calibration method associated with any of FIGS.11, 13, 15, and 16. For the sake of brevity, the combination of theshutter calibration method and sensor calibration methods associatedwith FIGS. 15 and 16 is expressively described. A person of ordinaryskill in the art would recognize that computations associated withcombinations of the calibration methods that are not explicitlydisclosed can be easily derived from the disclosure and are within thescope of the disclosure.

In some embodiments, when the shutter is closed, the calibrationquantities associated with the active sensors (e.g., equation (15),D_(sij)(0)) and calibration sensor (e.g., equation (20), D_(M)(0)) arecomputed. As such, equations (21) and (22) are combined to compute thequantity P_(ij), which is associated with shutter calibration, thecalibration sensor, and the calibration currents.

$\begin{matrix}{P_{ij} = {\left( {{D_{sij}(t)} - {D_{sij}(0)}} \right) - {\left( {{D_{iM}(t)} - {D_{iM}(0)}} \right)\frac{Z_{M}}{Z_{sij}}}}} & (23)\end{matrix}$

Equation (23) yields the term P_(ij), which is substantiallyproportional to changes in the sensor image (i.e., change in activesensor impedance, ΔZ_(ij)). For example, the sensors are bolometers andthe sensor image is incoming thermal radiation.

$\begin{matrix}{P_{ij} \cong {{- \frac{1}{\Delta \; I}}\frac{V_{biasi}}{Z_{ij}}\frac{\Delta \; Z_{ij}}{Z_{ij}}}} & (24)\end{matrix}$

In some embodiments, the combined shutter calibration, calibrationsensor, and calibration current method utilizes one calibration current.In these embodiments, the term P_(ij) is substantially proportional tochanges in the sensor image as follows:

$\begin{matrix}{P_{ij} \cong {{- g_{j}}\frac{V_{biasi}}{Z_{ij}}\frac{\Delta \; Z_{ij}}{Z_{ij}}}} & (25)\end{matrix}$

For the sake of brevity, terms described in other equations are notdescribed again.

FIG. 17 illustrates a method 1700 of sensor calibration, in accordancewith an embodiment. In some embodiments, method 1700 is included inmethod 1400.

Method 1700 includes electrically uncoupling a readout element from afirst calibration current source (step 1702). For example, referring toFIG. 16, the calibration current sources in calibration current circuit1630 are electrically uncoupled from the readout elements in sensorreadout circuit 1610.

The method 1700 includes electrically coupling a second readout elementto the calibration current source (step 1704). For example, referring toFIG. 16, one of the calibration current sources in calibration currentcircuit 1630 is electrically coupled to the calibration readout elementin calibration sensor circuit 1620.

The method 1700 includes measuring, with the second readout element, athird readout voltage caused by the calibration current (step 1706). Forexample, referring to FIG. 16, a readout voltage caused by thecalibration current source is measured with the calibration readoutelement.

The method 1700 includes electrically uncoupling the second readoutelement from the calibration current source (step 1708). For example,referring to FIG. 16, the calibration current sources in calibrationcurrent circuit 1630 is electrically uncoupled from the calibrationreadout element in calibration sensor circuit 1620.

The method 1700 includes electrically coupling a first terminal of acalibration sensor to a bias voltage node shared by a plurality ofactive sensors and the active sensor (step 1710). For example, referringto FIG. 16, the bias voltage node in sensor readout circuit 1610 iselectrically coupled to the calibration sensor in calibration sensorcircuit 1620.

The method 1700 includes electrically coupling a second terminal of thecalibration sensor to the second readout element (step 1712). Forexample, referring to FIG. 16, the calibration sensor in calibrationsensor circuit 1620 is electrically coupled to the calibration readoutelement in calibration sensor circuit 1620.

The method 1700 includes exposing a plurality of active sensors and theactive sensor to a sensor image (step 1714). For example, referring toFIG. 16, the plurality of active sensors in sensor readout circuit 1610is exposed to a sensor image.

The method 1700 includes shielding the calibration sensor from thesensor image (step 1716). For example, referring to FIG. 16, thecalibration sensor in calibration sensor circuit 1620 is shielded fromthe sensor image.

The method 1700 includes measuring, with the second readout element, afourth readout voltage of the calibration sensor (step 1718). Forexample, referring to FIG. 16, a readout voltage of the calibrationsensor in calibration sensor circuit 1620 is measured with calibrationreadout element in calibration sensor circuit 1620.

The method 1700 includes computing a second output. In some examples,the output is based on the third readout voltage and the fourth readoutvoltage (step 1720). For example, referring to FIG. 7 and equation (20),D(t) associated with the calibration sensor column is computed based onthe third and fourth readout voltages.

The method 1700 includes computing a difference. In some examples, thedifference is between (1) the first output and (2) the second outputweighted by a ratio between an impedance of the calibration sensor andan impedance of the active sensor (step 1722). For example, referring toFIG. 7 and equation (21), the term P_(ij) is computed based on theoutputs D_(sij)(t) and D_(iM)t).

FIG. 18A illustrates an exemplary sensor image. Due to pattern noises,horizontal noise patterns (an artifact of which is indicated with 1810)and vertical noise patterns (an artifact of which is indicated with1820) are present in sensor image 1800.

FIG. 18B illustrates an exemplary sensor image after the disclosedcalibration methods are used. By using the disclose calibration methods,sensor image 1850 is free of the horizontal and vertical noise patternsthat are observed in sensor image 1800.

Some MEMS systems utilize capacitive elements (e.g., sensors,capacitors) to store charge for readout. For example, x-ray imagingsystems can implement a two dimensional array of sensor pixels thateither (1) directly convert x-ray photons to electrons or (2) utilize ascintillator plate to convert x-ray photons into visible photons whichare then converted into photoelectrons by photodetectors that aresensitive to visible light. The amount of electrons or photoelectronsincident on a sensor pixel can be represented by a charge stored in anequivalent capacitor (e.g., the junction capacitance in a photodiode, apixel capacitor storing the photo-generated charge prior to readout).

In a typical x-ray imaging system, the charge of each sensor is readoutone row at a time; each column of sensors connects to a CTIA followed byan ADC. These architectures can require a large chip area and asignificant amount of power (e.g., 3-5 mW per channel) in order toachieve a desired sensitivity and resolution. A typical panel with over3,000 columns would require the same amount of CTIAs. In typical x-rayimaging systems, a CTIA can include a high gain op-amp and a relativelylarge capacitor (e.g., 1-5 pF) to match the x-ray pixel capacitance. Thehigh gain nature of the op-amp results in high power dissipation, whichscales with the number of columns.

Examples of the disclosure are directed toward MEMS sensor circuits andmethods that reduce chip size and power dissipation of traditional MEMSsystems, while reducing cost and increasing reliability and portability.In some embodiments herein, a sensor circuit includes a plurality ofsensors each configured to store a charge, a Sigma-Delta ADC configuredto receive the charge of each sensor, and a plurality of switches (eachswitch corresponding to a respective one of the plurality of sensors)configured to sequentially couple each of the plurality of sensors tothe Sigma-Delta ADC. In some embodiments, the sensor circuit does notinclude a CTIA. In some embodiments, the sensor circuit does not includea CTIA electrically positioned between the plurality of sensor pixelsand the Sigma-Delta ADC. In some instances, replacing a CTIA and an ADCwith a Sigma-Delta ADC reduces the readout element's power and areabecause the CTIA's high-gain op-amp and large capacitor of the CTIA areno longer in the circuit. If Sigma-Delta ADCs replace CTIAs for allsensor columns, then the power and area reduction would be scaled by thenumber of sensor columns (see above—in some instances, an imaging systemincludes 3,000 columns), which can provide significant power and areasavings without compromising sensitivity, resolution, or noise.Reduction of power and area can reduce cost and increase reliability andportability of a system (e.g., an x-ray imaging system, a CMOS imagingsystem, a CCD imaging system) that includes the sensor circuit.

FIG. 19A illustrates an exemplary sensor circuit 1900. As illustrated,the sensor circuit 1900 includes sensors 1902A-1902D, switches1904A-1904D, and ADC 1906. In some embodiments, the ADC 1906 is aSigma-Delta ADC that is configured to capture the charge of a selectedsensor. In some instances, when a sensor is selected for readout, thedischarge current from the selected sensor is a time-varying (e.g.,exponentially decaying, step) signal. In some embodiments, theSigma-Delta ADC is configured to receive the time-varying signal andcapture substantively the total charge of the selected sensor. In someinstances, a Sigma-Delta ADC can measure the total charge from thetime-varying signal with sufficient resolution to satisfy accuracyrequirements of a system. In some embodiments, a switch electricallycouples a respective sensor to the Sigma-Delta ADC when a row of therespective sensor is selected (e.g., switch 1904A electrically couplessensor 1902A, which corresponds to a specific row, to the Sigma-DeltaADC). In some embodiments, the sensor circuit does not include a CTIAelectrically positioned between the plurality of sensors and theSigma-Delta ADC. Some embodiments include a method of manufacturing thecircuit 1900.

In some embodiments, with respect to the sensor circuits described inFIGS. 1-18, the sensor circuit 1900 can be included in the describedreadout element in place of a CTIA. For example, readout element 102 or1002 includes ADC 1906.

It is understood that the capacitor symbols used to schematicallyrepresent the sensors are merely exemplary and not limiting. In someembodiments, the sensor 1902 is a sensor pixel. In some embodiments, thesensor 1902 is a sensor that receives radiation (e.g., X-ray) andconverts the radiation into charge. In some embodiments, a charge isstored in a sensor pixel while the sensor pixel is sensing, and thecapacitor is representative of the sensor pixel's charge storingoperation. For example, the sensor is an x-ray sensor photodiode, andthe sensor accumulates charge in response to exposure to x-rayradiation; the total accumulated charge is representative of the x-rayradiation level and the capacitor symbol is used to represent the chargestoring operation. As another example, the sensor is an x-ray sensorpixel that directly converts x-ray photons into electrons and theelectrons are stored in a capacitor (e.g., a pixel capacitor storing thephoto-generated charge prior to readout).

It is understood that the sensor circuit 1900 is merely exemplary. Insome embodiments, the sensor circuit 1900 represents one column ofsensors, and each sensor of the column belongs to a row (not shown) ofsensors; each sensor of a column is readout at a different row time(e.g., during different discharge time windows). In some embodiments, asecond sensor of the column is readout at a subsequent readout timeusing the same method described herein. For example, a first switchdecouples a first sensor from the Sigma-Delta ADC and a second switchelectrically couples a second sensor to the Sigma-Delta ADC during asecond row time (e.g., a second discharge time window).

In some instances, a Sigma-Delta ADC may be more suitable for receivingnear-DC signals. Therefore, converting the discharge signal into anear-DC signal (e.g., a constant current) would be more desirable whenincluding a Sigma-Delta ADC in a sensor circuit. A CTIA and asample-and-hold circuit may be included between the sensors and theSigma-Delta ADC to generate a near DC input for Sigma-Deltaanalog-to-digital conversion. This may be particularly advantageouswhere a signal directly readout from a sensor includes high peaks andrapid transitions. If the signal includes sharp features, ADC resolutionmay be lost as the signal may not be adequately captured by theSigma-Delta ADC. However, as discussed earlier, adding a CTIA for eachcolumn of a sensor array can increase readout power and area of thereadout chip.

In some embodiments, a CTIA and a sample-and-hold circuit are notincluded with a Sigma-Delta ADC. FIG. 19B illustrates an exemplarysensor circuit 1950. As illustrated, the sensor circuit 1950 includessensors 1952A-1952D, switches 1954A-1954D, ADC 1956, variable resistor1958, and control voltage 1960. In some embodiments, the ADC 1956 is aSigma-Delta ADC that is configured to capture a total charge of aselected sensor. In some instances, when a sensor is selected forreadout, the discharge current from the selected sensor is atime-varying (e.g., exponentially decaying, step) signal. In someembodiments, the variable resistor is configured to receive thetime-varying signal and output the total charge to the Sigma-Delta ADC.In some embodiments, the sensor circuit does not include a CTIAelectrically positioned between the plurality of sensors and theSigma-Delta ADC. In some embodiments, sensors 1952A-1952D correspond tosensors 1902A-1902D, switches 1954A-1954D correspond to switches1904A-1904D, and ADC 1956 correspond to ADC 1906. For the sake ofbrevity, elements similar to the ones described in FIG. 19A will not bedescribed again. Some embodiments include a method of manufacturing thecircuit 1950.

In some embodiments, with respect to the sensor circuits described inFIGS. 1-18, the sensor circuit 1950 can be included in the describedreadout element in place of a CTIA. For example, readout element 102 or1002 includes ADC 1956 and variable resistor 1958. In some embodiments,the variable resistor 1958 is electrically coupled to sensor 110, 112,1008, or 1010 and is controlled with the methods described herein tominimize the time-varying effects of self-heating.

Variable resistor 1958 in front of Sigma-Delta ADC allows the dischargeof a capacitor to be controlled during readout to reduce high peakamplitude and sharp features of an uncontrolled discharge signal. As anexample, during sensor readout (e.g., when the accumulated charge of anx-ray sensor photodiode discharges to readout circuitry), the dischargesignal can be controlled by varying the resistance of the variableresistor and matching a discharge time window with a row time of thereadout operation. In some embodiments, a row time is when a row ofsensors is readout.

In some embodiments, the variable resistor 1958 is implemented with aMOS transistor. In some embodiments, the resistance of the MOStransistor can be controlled with a control voltage 1960, which controlsthe transconductance of the transistor. It is understood that thetransistor symbol is used to represent the variable resistor 1958 andthe control voltage 1960 connected to the variable resistor are notlimiting; other implementations of the variable resistor 1958 can existwithout departing from the scope of the disclosure.

In some embodiments, the variable resistor 1958 is implemented with aweighted bank of resistors (not shown) in which discrete levels ofresistance can be achieved (e.g., each combination between resistors ofthe weighted bank of resistors is unique). For example, the weightedbank of resistors includes a plurality of resistors that can beselectively electrically coupled in parallel or series and a pluralityof corresponding switches. In some embodiments, each resistor has aunique value such that every different combination of resistors resultin a different total resistance (e.g., the resistors' values form a setof basis values that span a range of resistances from high to low in astepwise manner (e.g., by increments of 50 ohms from 5.05 k-ohms to 5ohms)).

As an example, the variable resistor 1958 is a MOS transistor. In thisexample, the beginning of the discharge time window matches thebeginning of a row time. During this time (e.g., t=0), the resistance ofthe variable resistor 1958 is at an initial resistance (e.g., R₀), andthe switch 1954 electrically couples the sensor 1952 to the variableresistor 1958. As time progresses during this discharge time window, theresistance of the variable resistor 1958 decreases from R₀. In thisexample, the resistance is decreased linearly from R₀ to zero by the endof the discharge time window (e.g., t=T). In some embodiments, T is 20microseconds. In some embodiments, T is 40 microseconds. In someembodiments, T is between 10 microseconds and 1 millisecond. In someembodiments, the discharge time window is several magnitudes longer thana rise time of a signal turning on a comparable MOS transistor.Therefore, reducing a resistance of the variable resistor 1958 over thedischarge time window may be different than merely turning on atransistor. For example, the discharge time window is in the microsecondrange and a rise time of the signal turning on the MOS transistor is innanosecond range. The resistance of the variable resistor 1958 can becalculated as follows:

$\begin{matrix}{{R(t)} = {R_{0}\left( {1 - \frac{t}{T}} \right)}} & (26)\end{matrix}$

In some embodiments, to control the resistance, the control voltage 1960is electrically coupled to the gate of the MOS transistor and increasesthe drain-to-source transconductance from low to high (i.e., resistancedecreases from high to low) (e.g., by increasing the gate voltage) foreach row time during readout. As described with the above equation, theresistance of the variable resistor is linearly decreasing from t=0 tot=T; the resistance at t=0 (e.g., R(0)) is R₀, the initial resistance,after t=0, the resistance is linearly decreasing as described withrespect to the equation, and the resistance (e.g., R(T)) approachessubstantially zero at t=T. In some embodiments, the variable resistor1958 is implemented with a weighted bank of resistors (not shown) inwhich discrete levels of resistance can be achieved (e.g., eachcombination between resistors of the weighted bank of resistors isunique). In these embodiments, R(t) can be approximated with a stepwisefunction of the resistors of the weighted bank.

Although equation (26) shows that, in an ideal case, R(t) reaches zeroat t=T, it is understood that components used to implement the variableresistor 1958 may not reach exactly zero resistance at an end of adischarge time window. In some embodiments, this resistance is thevariable resistor's lowest resistance. For example, if the variableresistor is a MOS transistor, then the lowest resistance is determinedby the conductance of the transistor (e.g., the transistor's “on”resistance). As another example, if the variable resistor is a weightedbank of resistors, the lowest resistance is achieved by electricallycoupling all the resistors of the bank in parallel.

The capacitance of the sensor 1902 can be represented with C. Thecurrent going into the variable resistor 1958 can be calculated asfollows:

$\begin{matrix}{{C\frac{dV}{dt}} = {- \frac{V}{R(t)}}} & (27)\end{matrix}$

By solving for V(t), the voltage across the variable resistor 1958 canbe calculated as a function of time:

$\begin{matrix}{{V(t)} = {{V(0)}\left\lbrack {1 - \frac{t}{T}} \right\rbrack}^{\frac{T - {CR}_{0}}{{CR}_{0}}}} & (28)\end{matrix}$

The current I(t) across the variable resistor 1958 can be expressed as:

$\begin{matrix}{{I(t)} = {\frac{{{V(0)}\left\lbrack {1 - \frac{t}{T}} \right\rbrack}^{\frac{T}{{CR}_{0}}}}{R_{0}\left( {1 - \frac{t}{T}} \right)} = \frac{{{V(0)}\left\lbrack {1 - \frac{t}{T}} \right\rbrack}^{\frac{T - {CR}_{0}}{{CR}_{0}}}}{R_{0}}}} & (29)\end{matrix}$

As shown with equation (29), if the discharge time window T is set to beequal to the initial time constant (e.g., T=C×R₀), the current outputcan be substantially constant (e.g., I=V(0)/R₀). Accordingly, in someembodiments, the R₀ of the variable resistor is determined by theeffective capacitance (e.g., 1-5 pF) of the sensor 1902 and thedischarge time window (e.g., row time). By converting the dischargecurrent to a constant current, a higher resolution can be achieved witha Sigma-Delta ADC (e.g., 16 bits conversion (14 effective numbers ofbits)) without the above-described larger area and power penalties.Therefore, if a row time is known or determined (e.g., by systemrequirements), and sensor capacitance is known, the variable resistorcan be designed to convert the discharge current into a constantcurrent. By converting to the constant current using the sensor circuit1950, resolution degradation due to high peak and/or sharp dischargesignals being inputted to a Sigma-Delta ADC can be reduced withoutincluding a CTIA. Although the term “constant current” is used in thisexample, it is understood that the converted current may includevariations that do not substantively reduce the resolution of aSigma-Delta ADC.

In some embodiments, at an end of a discharge time window, the charge ofa sensor may not be completely readout. In some embodiments, theremaining charge may be discharged into the Sigma-Delta ADC withadditional discharge paths. In some embodiments, the remaining chargemay be ignored without substantively affecting ADC measurement.

It is understood that the sensor circuit 1950 is merely exemplary. Insome embodiments, the sensor circuit 1950 represents one column ofsensors, and each sensor of the column belongs to a row of sensors; eachsensor of a column is being readout at a different row time (e.g., atdifferent discharge time windows). In some embodiments, a second sensorof the column is readout at a subsequent readout time using the samemethod described herein. For example, a second switch electricallycouples a second sensor to the variable resistor during a second rowtime (e.g., a second discharge time window), and the current of thesecond sensor is converted to be near-DC for the Sigma-Delta ADC usingthe methods described herein.

Although the sensor circuit 1950 is described with one variableresistor, it is understood that a column of sensors can be associatedwith more than one variable resistor.

In some embodiments, R₀ can be adjusted in response to an updated rowtime requirement. For example, a weighted bank of resistors can beadjusted to set a different initial resistance. As another example,initial resistance of the variable resistor can be adjusted beforereadout.

Although the above examples are sometimes described with reference tox-ray sensor photodiodes, it is understood that other types of sensorscan use a similar current-controlling circuit without departing from thescope of the disclosure. For example, the described circuits can be usedfor CMOS or CCD imaging systems.

FIG. 20A illustrates an exemplary sensor circuit 2000. The sensorcircuit includes sensors 2002A-2002B and readout circuits 2004A-2004B.In some embodiments, the readout circuits include readout circuits 1900or 1950. Because sensor circuit 2000 may not include a CTIA, the sensorcircuit can enjoy the herein-described power, area, and resolutionbenefits.

As illustrated, the sensors 2002 are organized by N columns 2002A to2002N. It is understood that “N” can be any number of columns. Asdescribed with respect to FIGS. 20A and 20B, “a column of sensors” is aplurality of sensors included in an array of sensors, arranged along afirst dimension of the array, and bounded by boundaries of the array.Each sensor of the plurality sensors belongs to a unique row along asecond dimension of the array. For example, as illustrated in FIG. 20A,the sensors 2002A and 2002B are spatially arranged by vertical columnsof sensors (e.g., columns 2002A to 2002N). As another example, a columnof sensor is electrically coupled to the Sigma-Delta ADC in circuits1900 and 1950. As illustrated, readout circuit 2004A is configured toreadout sensors 2002A, and readout circuit 2004B is configured toreadout sensors 2002B.

In some embodiments, the sensors are x-ray sensor photodiodes and arepart of an x-ray panel. In some embodiments, the sensors are part of aCMOS or CCD panel. In some embodiments, the readout circuits includeSigma-Delta ADCs. In some embodiments, the readout circuits do notinclude CTIA.

FIG. 20B illustrates an exemplary sensor circuit 2050. As illustrated,the sensors 2052 are organized by N columns 2056A to 2056N. The sensorcircuit includes sensors 2052A-2052D and readout circuits 2054A-2054D.In some embodiments, the sensors are x-ray sensor photodiodes and arepart of an x-ray panel having a same size as the panel described in FIG.20A. In some embodiments, the sensors are part of a CMOS or CCD panel.In some embodiments, the sensors are arranged in columns, and eachcolumn includes a first plurality of sensors connected to a firstreadout circuit and a second plurality of sensors connected to a secondreadout circuit. For example, sensors 2052A includes the first pluralityof sensors, and sensor 2052B includes the second plurality of sensors;the combination of sensors 2052A and 2052B correspond to sensors 2002Adescribed in FIG. 20A. As another example, column 2056A includes sensorfrom the first plurality of sensors and from the second plurality ofsensors. In these instances, the panel is split—the first plurality ofsensors includes a first half of the column sensors (as defined withrespect to FIG. 20A), and the second plurality of sensors includes asecond half of the column of sensors (e.g., the numbers of the first andsecond plurality of sensors are equal).

As illustrated, readout circuit 2004A is configured to readout sensors2002A, readout circuit 2004B is configured to readout sensors 2002B,readout circuit 2004C is configured to readout sensors 2002C, andreadout circuit 2004D is configured to readout sensors 2004D. In someembodiments, a column is readout bi-directionally. For example, thefirst and second pluralities of sensors are readout at a same time withreadout circuits 2004A and 2004B.

By reading different parts of a column at the same time, the parasiticcapacitance and resistance involved in the readout can be reduced. Forexample, the parasitic resistances and capacitances of column 2056A atthe inputs of the respective readout circuits are each halved. Byreducing the parasitic RC of a column, readout delays can be reduced.Additionally, by “breaking up” a column, row addressing complexity canbe reduced. For example, the total row addresses are reduced becausethere are half as many rows to readout for each readout ASIC (comparedthe arrangement in FIG. 20A).

In some embodiments, each of the readout circuits includes the sensorcircuit 1900 or 1950 (for brevity, they will not be described again) anda Sigma-Delta ADC corresponding to each of the plurality of sensors. Insome embodiments, the readout circuits do not include a CTIA. In thedescribed example, even though the number of readout circuits double ina split-panel configuration, the total readout circuits area and powerdissipation in the architecture described in FIG. 20B (e.g., readoutcircuits including Sigma-Delta ADC, readout circuits not including aCTIA) can be less than a non-split column system including one CTIA percolumn.

For example, for the 3072×3072 sensor array, due to area saving benefits(e.g., replacing CTIA, reduced addressing complexity, reduced columnloading), eight chips can be used in the split panel configuration(compared to twelve chips in a non-split column system including oneCTIA per column). By using sensor circuit 1900 or 1950 in the splitpanel configuration, system power can also be reduced because a CTIA notused for readout. For example, power per column can reduce from 3-5 mWper column to below 1 W per column. By splitting each column intosub-columns and reducing the parasitic RC electrically coupled to thereadout circuits (thereby reducing the propagation delay caused byparasitic effects), system performance can be improved. For example,parasitic capacitance of a column can range between 50-200 pF andparasitic resistance of a column can range between 1 kiloohm to 100kiloohms; if a column is split into half as described herein, theparasitic values would respectively be reduced by a factor of two.

FIGS. 21A-21D and table 1 show simulation results of the Sigma-DeltaADC, demonstrating conversion of time varying signals during a timeperiod. Table 1 shows Effective Number of Bits (ENOB) as a function ofOver Sampling Rate (OSR) and the input signal shapes described withrespect to FIGS. 21A-21D.

The curve in FIG. 21A shows random ideal constant inputs which generatethe ENOB shown in Table 1. The curve in FIG. 21B shows a current clampedsignal; a current clamp on a column forces a current inputted to the ADCto be constant until the charge is completely depleted, effectivelycreating pulse-width modulated inputs. The curve in FIG. 21C is anexponentially decaying signal that has a length of two RC timeconstants. In some embodiments, if the charge is not yet fully depletedafter two time constants, a low resistance switch is used to dischargethe remaining charge into the ADC. The curve in FIG. 21D is anexponentially decaying signal that has a length of five RC timeconstants. As shown in table 1, if the signal is allowed to decay forfive time constants, the ENOB is reduced compared to the two timeconstant signal. In some embodiments, the curves illustrated in FIGS.21C and 21D are representative of a sensor readout charge signal (e.g.,a sensor's total charge is discharged into the ADC). It is understoodthat the curves are presented for illustrative purposes and are notnecessarily drawn to scale.

TABLE 1 Effective Number of Bits (ENOB) as a function of OverSamplingRate (OSR) and input signal shape. Signal shape ENOB @ OSR = 128 ENOB @OSR = 256 Constant 13.0 15.3 Current clamped 6.4 7.4 (width modulation)2 Time constant decay 11.4 13.8 5 Time constant decay 9.4 11.2

In some embodiments, for a time-varying input signal, a digital filteris used with the Sigma-Delta ADC to improve performance. For example,the digital filter is used to process digital signals outputted fromcircuit 1900 or 1950. In some embodiments, the digital filter is aFinite Impulse Response (FIR) filter. The coefficients of the FiniteImpulse Response (FIR) filter can be chosen to reduce quantization errorand increase linearity. As an example, for the current clamped waveform(e.g., FIG. 21B), a FIR filter with the same value for all coefficients(e.g., a rectangular window) can be used to improve performance. Asanother example, for the exponentially decaying waveforms (e.g., FIGS.21C, FIG. 21D, a sensor discharge current), a Blackman or Hamming windowcan be used for the FIR filter to improve performance.

In some embodiments, a CEP (cyclic excitation process) effect isimplemented on amorphous-Silicon. This may increase the sensitivity ofthe sensor pixel. For example, an x-ray sensor pixel can be manufacturedwith an amorphous Si based nip (n-type/intrinsic/p-type) structure forindirect detection or an x-ray sensitive photoconductor (e.g., Selenium)with electrode structures to allow for photoelectrons to be addressed.In both cases, producing internal gain for the photoexcited (primary)charged carriers can reduce the x-ray radiation levels required toproduce detectable signals. CEP produces high gain without the highelectric fields (as may be the case for conventional avalanchemultiplication effects). In some instances, CEP devices can be used infor x-ray indirect detection. In some embodiments, CEP effect isharnessed in conjunction with x-ray photoconductors such as selenium.

FIG. 22 illustrates a method 2200 of manufacturing an electromechanicalsystem, in accordance with an embodiment. As non-limiting examples, theelectrochemical system could be associated with circuits 100, 300, 500,600, 800, 900, 1000, 1100, 1300, 1500, 1600, 1900, 1950, 2000, and 2050(and related methods). To manufacture an electromechanical system, allor some of the process steps in method 2200 could be used and used in adifferent order. As a non-limiting example, Step 2214 could be performedbefore Step 2212.

Method 2200 includes Step 2202, providing a substrate. In someembodiments, the substrate is made of glass. In some embodiments, thesubstrate is low temperature polycrystalline silicon. In someembodiments, the substrate is a borosilicate that contains additionalelements to fine tune properties. An example of a borosilicate is byCorning Eagle™, which produces an alkaline earth boro aluminosilicate (asilicate loaded with boron, aluminum, and various alkaline earthelements). Other variations are available from Asahi Glass™ or Schott™.

In some embodiments, a flat panel glass process is used to manufacturethe electromechanical system. In some embodiments, a liquid crystaldisplay (LCD) process is used to manufacture the electromechanicalsystem. In some embodiments, an OLED display process or an x-ray panelprocess is used. Employing a flat panel glass process may allow forincreased substrate sizes, thereby allowing for a higher number ofelectrochemical systems per substrate, which reduces processing costs.“Panel level” sizes can include 620 mm×750 mm, 680 mm×880 mm, 1100mm×1300 mm, 1300 mm×1500 mm, 1500 mm×1850 mm, 1950 mm×2250 mm, and 2200mm×2500 mm. Further, thin film transistors (TFTs) in panel levelmanufacturing can also reduce cost and so, for example, LCD-TFTprocesses can be beneficial.

Method 2200 includes Step 2204, adding MEMS to the substrate. AlthoughMEMS is used to describe the addition of structures, it should beappreciated that other structures could be added without deviating fromthe scope of this disclosure. In embodiments using panel levelprocessing, the MEMS structures may be added using an LCD-TFT process.

Step 2204 may be followed by optional Step 2216, sub-plating. Step 2216may be used when the substrate is larger than the processing equipmentused in subsequent steps. For example, if using a panel level process(such as LCD), some embodiments will include (at Step 2204) cutting thepanel into wafer sizes to perform further processing (using, forexample, CMOS manufacturing equipment). In other embodiments, the samesize substrate is used throughout method 2200 (i.e., Step 2216 is notused).

Method 2200 includes Step 2206, releasing the MEMS from the substrate.

Method 2200 includes Step 2208, post-release processing. Suchpost-release processing may prepare the MEMS structure for furtherprocess steps, such as planarization. In wafer-level processing,planarization can include chemical mechanical planarization. In someembodiments, the further process steps include etch back, where aphotoresist is spun onto the topography to generate a more planarsurface, which is then etched. Higher control of the etch time can yielda smoother surface profile. In some embodiments, the further processsteps include “spin on glass,” where glass-loaded organic binder is spunonto the topography and the result is baked to drive off organicsolvents, leaving behind a surface that is smoother.

Method 2200 includes Step 2210, vacuum encapsulation of the MEMSstructure, where necessary. Vacuum encapsulation may be beneficial toprolong device life.

Method 2200 includes Step 2212, singulation. Some embodiments mayinclude calibration and chip programming, which may take into accountthe properties of the sensors. Methods described herein may beadvantageous in glass substrate manufacturing processes becauseuniformity in glass lithography capabilities is limited. As a furtheradvantage, glass has a lower thermal conductivity and so a glasssubstrate can be a better thermal insulator; by manufacturing thinstructures separating a bolometer pixel from a glass substrate,embodiments herein may better serve to thermally isolate the glassbolometer pixel from the packaging environment.

Method 2200 includes Step 2214, attachment of a readout integratedcircuit (ROIC) and flex/PCB attachment. As non-limiting examples, thereadout circuits could be associated with circuits 100, 300, 500, 600,800, 900, 1000, 1100, 1300, 1500, 1600, 1900, 1950, 2000, and 2050 (andrelated methods). Processes and devices described herein may have thefurther advantage that the area required for signal processing can bemuch smaller than the sensing area which is dictated by the sensingphysics. Typically, sensors are integrated on top of CMOS circuitry, andarea driven costs lead to a technology node that is not optimal for thesignal processing task. Processes described herein can use a moresuitable CMOS and drive down the area required for signal processing,freeing the sensor from any area constraints by leveraging the low costof FPD (flat panel display) manufacturing. In some embodiments, the ROICis specifically designed for sensing a specific electromagneticwavelength (such as X-Rays, THz, LWIR).

FIG. 23 illustrates an exemplary sensor. In some embodiments, sensor2300 is manufactured using method 2200. Sensor 2300 includes glasssubstrate 2306, structure 2304 less than 250 nm wide coupled to glasssubstrate 2306, and a sensor pixel 2302 coupled to the structure 2304.In some embodiments of sensor 2300, structure 2304 is a hinge thatthermally separates the active area from the glass. In some embodiments,sensor 2300 receives an input current or charge and outputs an outputcurrent or charge based on the received radiation (e.g., the resistancebetween two terminals of the sensor changes in response to exposure toLWIR radiation).

In some embodiments, a sensor includes a glass substrate, a structuremanufactured from any of the methods described herein and coupled to theglass substrate, and a sensor pixel coupled to the structure.

In some embodiments, a sensor includes a MEMS or NEMS devicemanufactured by a LCD-TFT manufacturing process and a structuremanufactured by any of the methods described herein.

By way of examples, sensors can include resistive sensors and capacitivesensors. Bolometers can be used in a variety of applications. Forexample, long wave infra-red (LWIR, wavelength of approximately 8-14 μm)bolometers can be used in the automotive and commercial securityindustries. For example, LWIR bolometers with QVGA, VGA, and otherresolution. Terahertz (THz, wavelength of approximately 1.0-0.1 mm)bolometers can be used in security (e.g., airport passenger securityscreening) and medical (medical imaging). For example, THz bolometerswith QVGA resolution and other resolutions. Some electrochemical systemscan include X-Ray sensors or camera systems. Similarly, LWIR and THzsensors are used in camera systems. Some electromechanical systems areapplied in medical imaging, such as endoscopes and exoscopes. X-raysensors include direct and indirect sensing configurations.

Other electromechanical systems include scanners for light detection andranging (LIDAR) systems. For example, optical scanners where spatialproperties of a laser beam could be shaped (for, e.g., beam pointing).Electromechanical systems include inertial sensors (e.g., where theinput stimulus is linear or angular motion). Some systems may be used inbio sensing and bio therapeutic platforms (e.g., where biochemicalagents are detected).

In one aspect, a sensor readout circuit includes a readout element, afirst current source, a second current source, a voltage driver, areference sensor, and an active sensor. The readout element includes aninput. The voltage driver includes an output. The reference sensorincludes a first terminal and a second terminal; the first terminal iselectrically coupled to the first current source and the second terminalis electrically coupled to the output of the voltage driver. The activesensor includes a first terminal and a second terminal; the firstterminal is electrically coupled to the second current source and theinput of the readout element and the second terminal is electricallycoupled to the output of the voltage driver. The active sensor isconfigured for exposure to a sensor image.

In some aspects of the above circuit, the first current and the secondcurrent are constant.

In some aspects of the each of the above circuits, the voltage drivergenerates a bias voltage for the active sensor.

In some aspects of the each of the above circuits, the active sensor isfurther configured to change a current from the first terminal of theactive sensor to the input of the readout element when the active sensoris exposed to the sensor image.

In some aspects of the each of the above circuits, the active sensor isfurther configured to change an impedance of the active sensor when theactive sensor is exposed to the sensor image.

In some aspects of the each of the above circuits, the reference sensoris a reference bolometer pixel and the active sensor is an activebolometer pixel.

In some aspects of the each of the above circuits, the circuit furtherincludes a second reference sensor, a second active sensor, a firstswitch, a second switch, a third switch, and a fourth switch. The secondreference sensor includes a first terminal and a second terminal; thefirst terminal is electrically coupled to the first current source andthe second terminal is electrically coupled to the voltage driver. Thesecond active sensor includes a first terminal and a second terminal;the first terminal is electrically coupled to the second current sourceoutputting the second current and the second terminal is electricallycoupled to the output of the voltage driver. The second active sensor isconfigured to change the current from the first terminal to the input ofthe readout element. The first switch is configured to selectivelyelectrically couple the reference sensor to the first current source.The second switch is configured to selectively electrically couple theactive sensor to the second source. The third switch is configured toselectively electrically couple the second reference sensor to the firstcurrent source. The fourth switch is configured to selectivelyelectrically couple the second active sensor to the second currentsource.

In some aspects of the each of the above circuits, the circuit furtherincludes a CDS circuit that is configured to remove an offset.

In some aspects of the each of the above circuits, the voltage of thereadout element is proportional to an impedance difference between thereference sensor and the active sensor.

In some aspects of the each of the above circuits, the circuit furtherincludes an output of an op amp electrically coupled to the secondterminal of the reference sensor.

In some aspects of the each of the above circuits, the circuit furtherincludes a feedback element that is electrically coupled to the firstand second terminals of the reference sensor.

In some aspects of the each of the above circuits, the circuit furtherincludes a third reference sensor and a third current source. The thirdreference sensor includes a first terminal and a second terminal that iselectrically coupled to the output of the voltage driver. The thirdcurrent source is electrically coupled to the first terminal of thethird reference sensor, and is configured to output a seventh currentreflective of self-heating generated by the third reference sensor. Thevalue of the second current adjusts in accordance with the seventhcurrent.

In some aspects of the each of the above circuits, the circuit furtherincludes an ADC that is configured to sample the change of the currentfrom the first terminal to the input of the readout element.

In some aspects of the each of the above circuits, the first currentsource and the second current source that are configured to output anequal magnitude of current in a same direction relative to therespective first terminals.

In some aspects of the each of the above circuits, the readout elementincludes a CTIA.

In some aspects of the each of the above circuits, the first currentsource and the second current source are selected from the group of anathermal voltage source and resistor, a high-impedance athermaltransistor current source, and a Wilson current mirror.

In some aspects of the each of the above circuits, the circuit furtherincludes an amplifier that outputs to the second terminal of thereference sensor. The first terminal of the reference sensorelectrically couples to a negative input of the amplifier. The firstcurrent source is configured to generate a voltage drop across thenegative input and the output.

In some aspects of the each of the above circuits, the reference sensoris a reference bolometer pixel, and the active sensor is a bolometerpixel configured to detect LWIR radiation.

In some aspects of the each of the above circuits, the readout elementincludes a Sigma-Delta ADC.

In some aspects of the each of the above circuits, a first stage of theSigma-Delta ADC includes a CTIA.

In some aspects of the each of the above circuits, the reference sensoris shielded from a sensor image.

In some aspects of the each of the above circuits, the circuit furtherincludes a voltage follower electrically coupled between the output ofthe voltage driver and the second terminal of the active sensor.

In some aspects of the each of the above circuits, the circuit furtherincludes two or more current buffers, the two or more current buffersincluding a first current buffer electrically coupled between the firstcurrent source and the reference sensor and a second current bufferelectrically coupled between the second current source and the activesensor.

In some aspects of the each of the above circuits, the circuit furtherincludes a fifth switch configured to selectively electrically couplethe active sensor to the voltage driver.

In another aspect, a method of sensor readout includes: providing afirst current to a first terminal of a reference sensor; generating,from the first current, a voltage at a second terminal of the referencesensor; providing a second current to a first terminal of the activesensor; driving, at the voltage, a second terminal of an active sensor;exposing the active sensor to a sensor image; and measuring a thirdcurrent from the first terminal of the active sensor to an input of areadout element.

In some aspects of the above method, the first current and the secondcurrent are constant.

In some aspects of each of the above methods, the voltage is a biasvoltage for the active sensor.

In some aspects of each of the above methods, exposing the active sensorto the sensor image further includes changing the third current.

In some aspects of each of the above methods, exposing the active sensorto the sensor image further includes changing an impedance of the activesensor.

In some aspects of each of the above methods, the method furtherincludes: providing a fourth current to a first terminal of a secondreference sensor; generating, from the fourth current, a second voltageat a second terminal of the second reference sensor; providing a fifthcurrent to a first terminal of a second active sensor; driving, at thesecond voltage, a second terminal of the second active sensor; exposingthe second active sensor to the sensor image; and measuring a sixthcurrent from the first terminal of the second active sensor to the inputof a readout element.

In some aspects of each of the above method, the method includes:electrically uncoupling, from the reference sensor, a first currentsource providing the first current; coupling, to the second referencesensor, the first current source providing the fourth current;electrically uncoupling, from the active sensor, a second current sourceproviding the second current; and coupling, to the second active sensor,the second current source providing the fifth current.

In some aspects of each of the above methods, the method furtherincludes: determining an offset generated by the input of the readoutelement; and canceling the offset prior to measuring the current to theinput of the readout element.

In some aspects of each of the above methods, a voltage at an output ofthe readout element is proportional to an impedance difference betweenthe reference sensor and the active sensor.

In some aspects of each of the above methods, the voltage is driven byan op amp, and the first terminal of the reference sensor iselectrically coupled to a negative input of the op amp.

In some aspects of each of the above methods, the method furtherincludes feeding back from the second terminal of the reference sensorto the first terminal of the reference sensor using a feedback element.

In some aspects of each of the above methods, the method furtherincludes: providing a seventh current to a first terminal of a thirdreference sensor, the seventh current reflective of self-heatinggenerated by the third reference sensor; and adjusting a value of thesecond current in accordance with the seventh current.

In some aspects of each of the above methods, the method furtherincludes sampling a voltage generated by the current to the input of areadout element.

In some aspects of each of the above methods, the first current and thesecond current are equal in magnitude and in a same direction relativeto the respective first terminals of the reference sensor and activesensor.

In some aspects of each of the above methods, the method furtherincludes converting the third current to a readout voltage of thereadout element.

In some aspects of each of the above methods, the first current and thesecond current are each provided by current sources selected from thegroup of an athermal voltage source and resistor, a high-impedanceathermal transistor current source, and a Wilson current mirror.

In some aspects of each of the above methods, driving the secondterminal of the active sensor at the voltage further includes driving,from an output of a voltage driver, the second terminal of the referencesensor and the second terminal of the active sensor.

In some aspects of each of the above methods, the method furtherincludes causing a voltage drop across the reference sensor from thefirst current; generating the voltage using an amplifier outputting tothe second terminal of the reference sensor; and electrically couplingthe first terminal of the reference sensor to a negative terminal of theamplifier.

In some aspects of each of the above methods, the reference sensor is areference bolometer pixel and the active sensor is an active bolometerpixel.

In some aspects of each of the above methods, exposing the active sensorto the sensor image further includes exposing the active sensor to LWIRradiation.

In some aspects of each of the above methods, the readout elementincludes a Sigma-Delta ADC.

In some aspects of the above method, a first stage of the Sigma-DeltaADC includes a CTIA.

In some aspects of each of the above methods, the method furtherincludes exposing the reference sensor to an ambient condition common tothe reference sensor and active sensor; and shielding the referencesensor from the sensor image.

In some aspects of each of the above methods, driving the secondterminal of the active sensor at the voltage further includes bufferingbetween the second terminal of the active sensor and a voltage sourceproviding the voltage.

In some aspects of each of the above methods, the method furtherincludes: buffering the first current; and buffering the second thecurrent.

In another aspect, a method of manufacturing a sensor readout circuitincludes providing a readout element including an input; providing afirst current source; providing a second current source; providing avoltage driver including an output; providing a reference sensorincluding a first terminal and a second terminal; electrically couplingthe first terminal of the reference sensor to the first current source;electrically coupling the second terminal of the reference sensor to theoutput of the voltage driver; providing an active sensor including afirst terminal and a second terminal, the active sensor configured forexposure to a sensor image; electrically coupling the first terminal ofthe active sensor to the second current source and the input of thereadout element; and electrically coupling the second terminal of theactive sensor to the output of the voltage driver.

In some aspects of the above method of manufacturing, the first currentand the second current sources are constant current sources.

In some aspects of each of the above methods of manufacturing, thevoltage driver is configured to generate a bias voltage for the activesensor.

In some aspects of each of the above methods of manufacturing, theactive sensor is further configured to change a current from the firstterminal of the active sensor to the input of the readout element whenthe active sensor is exposed to the sensor image.

In some aspects of each of the above methods of manufacturing, theactive sensor is further configured to change an impedance of the activesensor when the active sensor is exposed to the sensor image.

In some aspects of each of the above methods of manufacturing, thereference sensor is a reference bolometer pixel and the active sensor isan active bolometer pixel.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing a second referencesensor including a first terminal and a second terminal; electricallycoupling the first terminal of the second reference sensor to the firstcurrent source; electrically coupling the second terminal of the secondreference sensor to the voltage driver; providing a second active sensorincluding a first terminal and a second terminal, the second activesensor configured for exposure to the sensor image; electricallycoupling the first terminal of the active sensor to the second currentsource; electrically coupling the second terminal of the active sensorto the output of the voltage driver, and the second active sensor isconfigured to change a current from the first terminal of the activesensor to the input of the readout element; and providing a first switchconfigured to selectively electrically couple the reference sensor tothe first current source; providing a second switch configured toselectively electrically couple the active sensor to the second currentsource; providing a third switch configured to selectively electricallycouple the second reference sensor to the first current source; andproviding a fourth switch configured to selectively electrically couplethe second active sensor to the second current source.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes providing a CDS circuitconfigured to remove an offset.

In some aspects of each of the above methods of manufacturing, thereadout element is configured to generate a voltage proportional to animpedance difference between the reference sensor and the active sensor.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing an op amp; andelectrically coupling an output of an op amp to the second terminal ofthe reference sensor.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing a feedback element;and electrically coupling the feedback element to the first and secondterminals of the reference sensor.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing a third referencesensor including a first terminal and a second terminal; electricallycoupling the second terminal of the third reference sensor to the outputof the voltage driver; providing a third current source configured tooutput a seventh current reflective of self-heating generated by thethird reference sensor, where a value of the second current adjusts inaccordance with the seventh current; and electrically coupling the thirdcurrent source to the first terminal of the third reference sensor.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes providing an ADC configured tosample the change of the current from the first terminal to the input ofthe readout element.

In some aspects of each of the above methods of manufacturing, the firstcurrent source and the second current source are configured to output anequal magnitude of current in a same direction relative to therespective first terminals.

In some aspects of each of the above methods of manufacturing, thereadout element includes a CTIA.

In some aspects of each of the above methods of manufacturing, the firstcurrent source and the second current source are selected from the groupof an athermal voltage source and resistor, a high-impedance athermaltransistor current source, and a Wilson current mirror.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing an amplifieroutputting to the second terminal of the reference sensor; andelectrically coupling the first terminal of the reference sensor to anegative input of the amplifier, where the first current source isconfigured to generate a voltage drop across the negative input and theoutput.

In some aspects of each of the above methods of manufacturing, thereference sensor is a reference bolometer pixel and the active sensor isa bolometer pixel configured to detect LWIR radiation.

In some aspects of each of the above methods of manufacturing, thereadout element includes a Sigma-Delta ADC.

In some aspects of the above method of manufacturing, a first stage ofthe Sigma-Delta ADC includes a CTIA.

In some aspects of each of the above methods of manufacturing, thereference sensor is shielded from a sensor image.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing a voltage follower;and electrically coupling the voltage follower between the output of thevoltage driver and the second terminal of the active sensor.

In some aspects of each of the above methods of manufacturing, themethod of manufacturing further includes: providing two or more currentbuffers including a first current buffer and a second current buffer;electrically coupling the first current buffer between the first currentsource and the reference sensor; and electrically coupling the secondcurrent buffer between the second current source and the active sensor.

In one aspect, a sensor circuit includes: a plurality of active sensorsexposed to a sensor image and sharing a bias voltage node; a calibrationreadout element; and a calibration sensor shielded from the sensor imageand including a first terminal electrically coupled to the bias voltagenode and a second terminal electrically coupled to the calibrationreadout element.

In some aspects of the above circuit, an impedance of the calibrationsensor is the same as an impedance of an active sensor of the pluralityof active sensors, and where an electrical carrier count of thecalibration sensor is greater than an electrical carrier count of theactive sensor.

In some aspects of the above circuits, the sensor circuit furtherincludes: a readout element corresponding to an active sensor of theplurality of active sensors and configured to measure a readout voltageof the active sensor, where the calibration readout element isconfigured to measure a readout voltage of the calibration sensor, andthe sensor circuit is electrically coupled to: a processor; and a memoryincluding instructions, which when executed by the processor, cause theprocessor to perform a method that includes: receiving the readoutvoltage of the active sensor; receiving the readout voltage of thecalibration sensor; and computing a difference between (1) the readoutvoltage of the active sensor and (2) the readout voltage of thecalibration sensor weighted by a ratio between an impedance of thecalibration sensor and an impedance of the active sensor.

In some aspects of the above circuits, the ratio is one.

In some aspects of the above circuits, the ratio is temperatureindependent.

In some aspects of the above circuits, the sensor circuit furtherincludes: a readout element corresponding to an active sensor of theplurality of active sensors and configured to measure a readout voltageof the active sensor, where the sensor circuit is electrically coupledto: a processor; and a memory including instructions, which whenexecuted by the processor, cause the one or more processors to perform amethod that includes: receiving a first readout voltage corresponding toa closed shutter; receiving a second readout voltage corresponding to anopened shutter; and computing a difference proportional to an impedancedifference of the active sensor caused by the sensor image between (1)the first readout voltage and (2) the second readout voltage.

In some aspects of the above circuits, the plurality of readout elementsincludes a plurality of ADCs.

In some aspects of the above circuits, the calibration sensor and theplurality of active sensors are made from materials having a samethermal coefficient of resistance (TCR).

In some aspects of the above circuits, the plurality of active sensorsincludes a plurality of columns of active sensors, the circuit furtherincludes: a plurality of current sources, where a current source of theplurality of current sources is electrically coupled to the secondterminal of the calibration sensor and the calibration readout element;and a plurality of readout elements, where each of the plurality ofcolumns of active sensors is electrically coupled to: a correspondingcurrent source of the plurality of current sources at a correspondingreadout node, and a corresponding readout element of the plurality ofreadout elements at the corresponding readout node.

In some aspects of the above circuits, the calibration readout elementincludes an analog-to-digital converter (ADC).

In some aspects of the above circuits, the plurality of active sensorsand the calibration sensor are bolometers, and the sensor image is athermal image.

In another aspect, a sensor circuit includes: a calibration currentsource providing a calibration current; an active sensor; a readoutelement; a first switch configured to selectively electrically couplethe active sensor to the readout element; and a second switch configuredto selectively electrically couple the calibration current source to thereadout element.

In some aspects of the above circuits, the second switch is configuredto electrically uncouple the calibration current source from the firstreadout element when the first switch electrically couples the activesensor to the readout element, and the first switch is configured toelectrically uncouple the active sensor from the first readout elementwhen the second switch electrically couples the calibration current tothe readout element, and the sensor circuit is electrically coupled to:a processor; and a memory including instructions, which when executed bythe processor, cause the processor to perform a method including:receiving a first readout voltage of the active sensor; receiving asecond readout voltage caused by the calibration current; and computingan output proportional to a readout current of the active sensor basedon (1) the first readout voltage and (2) the second readout voltage.

In some aspects of the above circuits, the sensor circuit furtherincludes: a plurality of active sensors including the active sensor; anda plurality of readout elements including the first and second readoutelements, each of the plurality of readout elements electrically coupledto a respective active sensor of the plurality of active sensors, wherethe method further includes: receiving, from a readout element of theplurality of readout elements, a first readout voltage of the respectiveactive sensor; receiving a second readout voltage caused by thecalibration current on a respective readout element; and computing arespective output proportional to a readout current of the respectiveactive sensor based on (1) the readout voltage of the respective sensorand (2) the second readout voltage caused by the calibration current onthe respective readout element.

In some aspects of the above circuits, the sensor circuit furtherincludes a second active sensor belonging to a same column as the firstactive sensor, where the method further includes, after computing thefirst output: receiving a third readout voltage of the second activesensor; and computing a second output proportional to a readout currentof the fourth active sensor based on (1) the third readout voltage and(2) the second readout voltage caused by the calibration current.

In some aspects of the above circuits, a time between successivereceipts of the second readout voltage on the same column caused by thecalibration current is a calibration period.

In some aspects of the above circuits, the calibration period is onesecond.

In some aspects of the above circuits, the calibration period is basedon a drift of the readout element.

In some aspects of the above circuits, different rows are readout duringthe successive receipts of the second readout voltage.

In some aspects of the above circuits, the sensor circuit furtherincludes: a second calibration current source; a third switch configuredto selectively electrically couple the first calibration current sourceto the readout element; and a fourth switch configured to selectivelyelectrically couple the second calibration current source to the readoutelement, and where when the third switch electrically uncouples thereadout element from the first calibration current source: the fourthswitch is configured to electrically couple the readout element to thesecond calibration current source, and the method further includesreceiving a third readout voltage caused by the second calibrationcurrent; and the output is further based on the third readout voltagecaused by the second calibration current.

In some aspects of the above circuits, the readout element includes anADC.

In some aspects of the above circuits, the sensor circuit iselectrically coupled to: a processor; and a memory includinginstructions, which when executed by the processor, cause the one ormore processors to perform a method that includes: receiving a firstreadout voltage corresponding to a closed shutter; receiving a secondreadout voltage corresponding to an opened shutter; and computing adifference proportional to an impedance difference of the first activesensor caused by a sensor image between (1) the first readout voltageand (2) the second readout voltage.

In some aspects of the above circuits, the active sensor is a bolometerexposed to a thermal scene.

In some aspects of the above circuits, the active sensor is exposed to asensor image and shares a bias voltage node with a plurality of activesensors, and the sensor circuit further includes: a second readoutelement; and a calibration sensor shielded from the sensor image andincluding a first terminal electrically coupled to the bias voltage nodeand a second terminal electrically coupled to the second readoutelement.

In another aspect, a method of calculating a calibrated voltage in asensor circuit includes: electrically coupling a first terminal of acalibration sensor to a bias voltage node shared by a plurality ofactive sensors; electrically coupling a second terminal of thecalibration sensor to a calibration readout element; exposing theplurality of active sensors to a sensor image; shielding the calibrationsensor from the sensor image; measuring, with a readout element, areadout voltage of an active sensor of the plurality of active sensors;measuring, with the calibration readout element, a readout voltage ofthe calibration sensor; and computing the calibrated voltage as adifference between (1) the readout voltage of the active sensor and (2)the readout voltage of the calibration sensor weighted by a ratiobetween an impedance of the calibration sensor and an impedance of theactive sensor.

In some aspects of the above circuit, the impedance of the calibrationsensor is the same as the impedance of the active sensor, and where anelectrical carrier count of the calibration sensor is greater than anelectrical carrier count of the active sensor.

In some aspects of the above circuits, the ratio is one.

In some aspects of the above circuits, the ratio is temperatureindependent.

In some aspects of the above circuits, the calibration sensor and theactive sensor are made from materials having a same TCR.

In some aspects of the above circuits, the method further includes:electrically coupling a current source of a plurality of current sourcesto the second terminal of the calibration sensor and to the calibrationreadout element; electrically coupling a column of a plurality ofcolumns of active sensors to the readout element, the column of activesensors including the active sensor; and electrically coupling a secondcurrent source of the plurality of current sources to the readoutelement.

In some aspects of the above circuits, the method further includes:closing a shutter; measuring, with the readout element, a first readoutvoltage corresponding to the closed shutter; and measuring, with thecalibration readout element, a second readout voltage corresponding tothe closed shutter; and after computing the calibrated voltage,computing a second difference between (1) the calibrated voltage and adifference between (2a) the first readout voltage and (2b) the secondreadout voltage weighted by the ratio, where the second difference is ashutter calibrated voltage.

In some aspects of the above circuits, the calibration readout elementincludes an ADC.

In some aspects of the above circuits, the readout element includes anADC.

In some aspects of the above circuits, the plurality of active sensorsand the calibration sensor are bolometers, and the sensor image is athermal image.

Some aspects include a method of manufacturing the above circuits.

In another aspect, a method of calculating an output in a sensor circuitincludes: electrically coupling a readout element to an active sensor;measuring, with the readout element, a first readout voltage of theactive sensor; electrically uncoupling the readout element from theactive sensor; electrically coupling a calibration current to thereadout element; measuring, with the readout element, a second readoutvoltage caused by the calibration current; and computing the outputbased on (1) the first readout voltage and (2) the second readoutvoltage, the output proportional to a readout current of the activesensor.

In some aspects of the above method, the method further includes:electrically coupling a respective active sensor of a plurality ofactive sensors to a readout element of a plurality of readout elements;measuring, with the respective readout element, a first readout voltageof the respective active sensor; electrically uncoupling the respectivereadout element from the respective active sensor; electrically couplingthe calibration current to the respective readout element; measuring,with the respective readout element, a second readout voltage caused bythe calibration current on the respective readout element; and computingan output proportional to a readout current of the respective activesensor based on (1) the first readout voltage of the respective activesensor and (2) the second readout voltage caused by the calibrationcurrent.

In some aspects of the above methods, the method further includes, aftercomputing the first output: electrically uncoupling the calibrationcurrent source from the readout element; electrically coupling thereadout element to a second active sensor, the second active sensorbelonging to a same column as the first active sensor; measuring, withthe readout element, a third readout voltage of the second activesensor; and computing a second output proportional to a readout currentof the second active sensor based on (1) the third readout voltage and(2) the second readout voltage caused by the calibration current.

In some aspects of the above methods, a time between successivemeasurements of the second readout voltage on the same column caused bythe calibration current is a calibration period.

In some aspects of the above methods, the calibration period is onesecond.

In some aspects of the above methods, the calibration period is based ona drift of the readout element.

In some aspects of the above methods, different rows are readout duringthe successive measurements of the second readout voltage.

In some aspects of the above methods, the method further includes:electrically uncoupling the readout element from the first calibrationcurrent source; electrically coupling the readout element to a secondcalibration current source; and measuring, with the readout element, athird readout voltage caused by the second calibration current on thereadout element, where the output is further based on the third readoutvoltage caused by the second calibration current.

In some aspects of the above methods, the readout element includes anADC.

In some aspects of the above methods, the method further includes:closing a shutter; computing the output corresponding to a closedshutter; and computing a difference proportional to an impedancedifference of the active sensor caused by a sensor image between (1) theoutput corresponding to an opened shutter and (2) the outputcorresponding to the closed shutter.

In some aspects of the above methods, the active sensor is a bolometerexposed to a thermal scene.

In some aspects of the above methods, the method further includes:electrically uncoupling the readout element from the calibration currentsource; electrically coupling a second readout element to thecalibration current source; measuring, with the second readout element,a third readout voltage caused by the calibration current; electricallyuncoupling the second readout element from the calibration currentsource; electrically coupling a first terminal of a calibration sensorto a bias voltage node shared by a plurality of active sensors and theactive sensor; electrically coupling a second terminal of thecalibration sensor to the second readout element; exposing the pluralityof active sensors and the active sensor to a sensor image; shielding thecalibration sensor from the sensor image; measuring, with the secondreadout element, a fourth readout voltage of the calibration sensor;computing a second output based on the third readout voltage and thefourth readout voltage; and computing a difference between (1) the firstoutput and (2) the second output weighted by a ratio between animpedance of the calibration sensor and an impedance of the activesensor.

In one aspect, a sensor circuit, includes: a plurality of sensor pixels,each configured to store a charge; a Sigma-Delta ADC configured toreceive the charge of each sensor; and a plurality of switchesconfigured to sequentially couple each of the plurality of sensor pixelsto the Sigma-Delta ADC, each switch corresponding to a respective one ofthe plurality of sensor pixels.

In some aspects of the above circuit, the sensor circuit does notinclude a CTIA electrically positioned between the plurality of sensorpixels and the Sigma-Delta ADC.

In some aspects of the above circuits, the sensor circuit furtherincludes a variable resistor electrically positioned between theplurality of sensors and the Sigma-Delta ADC, wherein the plurality ofswitches are configured to sequentially couple each of the plurality ofsensor pixels to the variable resistor.

In some aspects of the above circuits, the variable resistor has alinearly decreasing resistance during a discharge time window; thevariable resistor is at a lowest resistance at an end of the dischargetime window; and the variable resistor has a resistance higher than thelowest resistance between the beginning and the end of the dischargetime window.

In some aspects of the above circuits, the variable resistor is a MOStransistor; and the initial resistance, the linearly decreasingresistance, and the lowest resistance of the MOS transistor arecontrolled with a control voltage electrically coupled to the MOStransistor.

In some aspects of the above circuits, the discharge time window isbetween 10 microseconds and 1 millisecond.

In some aspects of the above circuits, during the first discharge timewindow, a first switch electrically couples a first sensor pixel and theSigma-Delta ADC; during a second discharge time window, a second switchelectrically couples a second sensor pixel and the Sigma-Delta ADC; andthe first and second discharge time windows correspond to readout timesof the first and second sensor pixels.

In some aspects of the above circuits, during the discharge time window,a constant current of the variable resistor is an initial voltage of thevariable resistor divided by the initial resistance.

In some aspects of the above circuits, a switch electrically couples arespective sensor pixel and the variable resistor during a respectivedischarge time window, the discharge time window equal to a capacitanceof the sensor pixel multiplied by an initial resistance of the variableresistor.

In some aspects of the above circuits, the variable resistor includes aweighted bank of resistors; the weighted bank of resistors include aplurality of resistors selectively electrically coupled in parallel orin series; and resistances of combinations of the selective electricallycoupled resistors include an initial resistance at the beginning of adischarge time window, a linearly decreasing resistance, and a lowestresistance.

In some aspects of the above circuits, a sensor pixel includes an x-raysensor photodiode and the charge is indicative of the x-ray sensorphotodiode's exposure to x-ray.

In some aspects of the above circuits, a sensor pixel includes a storagecapacitor storing the charge and the sensor pixel's exposure to x-raygenerates the charge stored in the storage capacitor.

In some aspects of the above circuits, the sensor circuit furtherincludes a second plurality of sensor pixels and a second Sigma-DeltaADC, wherein the second plurality of sensor pixels are configured tosequentially couple to the second Sigma-Delta ADC and the first andsecond pluralities of sensor pixels belong to a same column.

In some aspects of the above circuits, numbers of the first and secondplurality of sensor pixels are equal.

In some aspects of the above circuits, at a first row time, a firstsensor pixel of the first plurality of sensor pixels and a second sensorpixel of the second plurality of sensor pixels are simultaneouslyreadout.

In some aspects of the above circuits, an input current to theSigma-Delta ADC is constant.

In some aspects of the above circuits, the sensor circuit furtherincludes a digital filter configured to receive a signal from theSigma-Delta ADC.

Some aspects include a method of manufacturing the above circuits.

In one aspect, a sensor circuit includes a plurality of sensor pixels, aSigma-Delta ADC, and a plurality of switches, each switch correspondingto a respective one of the plurality of sensor pixels; a method ofreadout of the sensor circuit includes: storing respective charges ineach of the plurality of sensor pixels; sequentially electricallycoupling, using the plurality of switches, each of the plurality ofsensor pixels to the Sigma-Delta ADC; and sequentially receiving, at theSigma-Delta ADC, the respective charge of each sensor pixel.

In some aspects of the above method, the sensor circuit does not includea CTIA electrically positioned between the plurality of sensor pixelsand the Sigma-Delta ADC and the respective charge of each sensor pixelis not received by the CTIA.

In some aspects of the above methods, the sensor circuit furtherincludes a variable resistor electrically positioned between theplurality of sensor pixels and the Sigma-Delta ADC and the methodfurther includes sequentially electrically coupling, using the pluralityof switches, each of the plurality of sensor pixels to the Sigma-DeltaADC further includes sequentially electrically coupling, using theplurality of switches, the each of the plurality of sensor pixels to thevariable resistor.

In some aspects of the above methods, the method further includeslinearly decreasing a resistance of the variable resistor during adischarge time window, wherein: the variable resistor is at a lowestresistance at an end of the discharge time window; and the variableresistor has a resistance higher than the lowest resistance between thebeginning and the end of the discharge time window.

In some aspects of the above methods, the variable resistor is a MOStransistor electrically coupled to a control voltage and linearlydecreasing the resistance of the variable resistor further includesdriving the MOS transistor with the control voltage to generate theinitial resistance, the linearly decreasing resistance, and the lowestresistance.

In some aspects of the above methods, the discharge time window isbetween 10 microseconds and 1 millisecond.

In some aspects of the above methods, sequentially electricallycoupling, using the plurality of switches, each of the plurality ofsensor pixels to the Sigma-Delta ADC further includes: during the firstdischarge time window, electrically coupling a first switch to a firstsensor pixel and the Sigma-Delta ADC; during a second discharge timewindow, electrically coupling a second switch to a second sensor pixeland the Sigma-Delta ADC, wherein the first and second discharge timewindows correspond to readout times of the first and second sensorpixels.

In some aspects of the above methods, during the discharge time window,a constant current of the variable resistor is an initial voltage of thevariable resistor divided by the initial resistance.

In some aspects of the above methods, sequentially electricallycoupling, using the plurality of switches, each of the plurality ofsensor pixels to the Sigma-Delta ADC further includes electricallycoupling a switch to a respective sensor pixel and the variable resistorduring a respective discharge time window; and the discharge time windowis equal to a capacitance of the sensor pixel multiplied by an initialresistance of the variable resistor.

In some aspects of the above methods, the variable resistor includes aweighted bank of resistors; the weighted bank of resistors include aplurality of resistors selectively electrically coupled in parallel orin series; and the method further includes linearly decreasingresistances of combinations of the plurality of resistors, from aninitial resistance at a beginning of a discharge time window to a lowestresistance at an end of the discharge time window, by selectiveelectrically coupling the resistors.

In some aspects of the above methods, a sensor pixel includes an x-raysensor photodiode and the charge is indicative of the x-ray sensorphotodiode's exposure to x-ray.

In some aspects of the above methods, storing respective charges in eachof the plurality of sensor pixels further includes: exposing the each ofthe plurality of sensor pixels to x-ray and generating the respectivecharge; and storing the respective charges in a storage capacitor of theeach of the plurality of sensor pixels.

In some aspects of the above methods, the sensor circuit furtherincludes a second plurality of sensor pixels belonging to a same columnas the first plurality of sensor pixels, a second plurality of switches,and a second Sigma-Delta ADC, the method further includes: sequentiallyelectrically coupling, using the second plurality of switches, each ofthe plurality of sensor pixels to the second Sigma-Delta ADC; andsequentially receiving, at the second Sigma-Delta ADC, the respectivecharge of each sensor pixel of the second plurality of sensor pixels.

In some aspects of the above methods, numbers of the first and secondplurality of sensor pixels are equal.

In some aspects of the above methods, at a first row time: the firstSigma-Delta ADC receives a first respective charge of a first sensorpixel of the first plurality of sensor pixels; and the secondSigma-Delta ADC receives a second respective charge of a second sensorpixel of the second plurality of sensor pixels.

In some aspects of the above methods, the Sigma-Delta ADC receives aconstant current.

Generally, as used herein, the term “substantially” is used to describeelement(s) or quantit(ies) ideally having an exact quality (e.g., fixed,the same, uniformed, equal, similar, proportional), but practicallyhaving qualities functionally equivalent to the exact quality. Forexample, an element or quantity is described as being substantiallyfixed or uniformed can deviate from the fixed or uniformed value, aslong as the deviation is within a tolerance of the system (e.g.,accuracy requirements, etc.). As another example, two elements orquantities described as being substantially equal can be approximatelyequal, as long as the difference is within a tolerance that does notfunctionally affect a system's operation.

Likewise, although some elements or quantities are described in anabsolute sense without the term “substantially”, it is understood thatthese elements and quantities can have qualities that are functionallyequivalent to the absolute descriptions. For example, in someembodiments, a ratio is described as being one. However, it isunderstood that the ratio can be greater or less than one, as long asthe ratio is within a tolerance of the system (e.g., accuracyrequirements, etc.).

As used herein, “substantially the same” sensors produce a similarresponse to a given stimulus. For example, “substantially the same”bolometers produce a similar resistance change for a given temperaturechange.

Although the disclosed embodiments have been fully described withreference to the accompanying drawings, it is to be noted that variouschanges and modifications will become apparent to those skilled in theart. Such changes and modifications are to be understood as beingincluded within the scope of the disclosed embodiments as defined by theappended claims.

The terminology used in the description of the various describedembodiments herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used in thedescription of the various described embodiments and the appendedclaims, the singular forms “a”, “an,” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

We claim:
 1. A sensor readout circuit, comprising: a readout elementcomprising an input; a first current source; a second current source; avoltage driver comprising an output; a reference sensor comprising afirst terminal and a second terminal, the first terminal electricallycoupled to the first current source and the second terminal electricallycoupled to the output of the voltage driver; and an active sensorcomprising a first terminal and a second terminal, the first terminalelectrically coupled to the second current source and the input of thereadout element and the second terminal electrically coupled to theoutput of the voltage driver, wherein the active sensor is configuredfor exposure to a sensor image.
 2. The circuit of claim 1, wherein thefirst current and the second current are constant.
 3. The circuit ofclaim 1, wherein the voltage driver generates a bias voltage for theactive sensor.
 4. The circuit of claim 1, wherein the active sensor isfurther configured to change a current from the first terminal of theactive sensor to the input of the readout element when the active sensoris exposed to the sensor image.
 5. The circuit of claim 1, wherein theactive sensor is further configured to change an impedance of the activesensor when the active sensor is exposed to the sensor image.
 6. Thecircuit of claim 1, wherein the reference sensor is a referencebolometer pixel and the active sensor is an active bolometer pixel. 7.The circuit of claim 1, further comprising: a second reference sensorcomprising a first terminal and a second terminal, the first terminalelectrically coupled to the first current source and the second terminalelectrically coupled to the voltage driver; a second active sensorcomprising a first terminal and a second terminal, the first terminalelectrically coupled to the second current source outputting the secondcurrent and the second terminal electrically coupled to the output ofthe voltage driver, and wherein the second active sensor is configuredto change the current from the first terminal to the input of thereadout element; and a first switch configured to selectivelyelectrically couple the reference sensor to the first current source; asecond switch configured to selectively electrically couple the activesensor to the second current source; a third switch configured toselectively electrically couple the second reference sensor to the firstcurrent source; and a fourth switch configured to selectivelyelectrically couple the second active sensor to the second currentsource.
 8. The circuit of claim 1, further comprising a correlateddouble sampling (CDS) circuit configured to remove an offset.
 9. Thecircuit of claim 1, wherein a voltage of the readout element isproportional to an impedance difference between the reference sensor andthe active sensor.
 10. The circuit of claim 1, further comprising anoutput of an op amp electrically coupled to the second terminal of thereference sensor.
 11. The circuit of claim 1, further comprising afeedback element electrically coupled to the first and second terminalsof the reference sensor.
 12. The circuit of claim 1, further comprising:a third reference sensor comprising a first terminal and a secondterminal electrically coupled to the output of the voltage driver; and athird current source electrically coupled to the first terminal of thethird reference sensor, and configured to output a seventh currentreflective of self-heating generated by the third reference sensor,wherein a value of the second current adjusts in accordance with theseventh current.
 13. The circuit of claim 1, further comprising an ADCconfigured to sample the change of the current from the first terminalto the input of the readout element.
 14. The circuit of claim 1, whereinthe first current source and the second current source are configured tooutput an equal magnitude of current in a same direction relative to therespective first terminals.
 15. The circuit of claim 1, wherein thereadout element comprises a capacitive transimpedance amplifier (CTIA).16. The circuit of claim 1, wherein the first current source and thesecond current source are selected from the group of an athermal voltagesource and resistor, a high-impedance athermal transistor currentsource, and a Wilson current mirror.
 17. The circuit of claim 1, furthercomprising an amplifier outputting to the second terminal of thereference sensor, wherein the first terminal of the reference sensorelectrically couples to a negative input of the amplifier and the firstcurrent source is configured to generate a voltage drop across thenegative input and the output.
 18. The circuit of claim 1, wherein thereference sensor is a reference bolometer pixel and the active sensor isa bolometer pixel configured to detect Long Wavelength Infrared (“LWIR”)radiation.
 19. The circuit of claim 1, wherein the readout elementcomprises a Sigma-Delta ADC.
 20. The circuit of claim 19, wherein afirst stage of the Sigma-Delta ADC comprises a CTIA.
 21. The circuit ofclaim 1, wherein the reference sensor is shielded from a sensor image.22. The circuit of claim 1, further comprising a voltage followerelectrically coupled between the output of the voltage driver and thesecond terminal of the active sensor.
 23. The circuit of claim 1,further comprising two or more current buffers, the two or more currentbuffers including a first current buffer electrically coupled betweenthe first current source and the reference sensor and a second currentbuffer electrically coupled between the second current source and theactive sensor.
 24. A method of sensor readout, comprising: providing afirst current to a first terminal of a reference sensor; generating,from the first current, a voltage at a second terminal of the referencesensor; providing a second current to a first terminal of the activesensor; driving, at the voltage, a second terminal of an active sensor;exposing the active sensor to a sensor image; and measuring a thirdcurrent from the first terminal of the active sensor to an input of areadout element.
 25. The method of claim 24, wherein the first currentand the second current are constant.
 26. The method of claim 24, whereinthe voltage is a bias voltage for the active sensor.
 27. The method ofclaim 24, wherein exposing the active sensor to the sensor image furthercomprises changing the third current.
 28. The method of claim 24,wherein exposing the active sensor to the sensor image further compriseschanging an impedance of the active sensor.
 29. The method of claim 24,further comprising: providing a fourth current to a first terminal of asecond reference sensor; generating, from the fourth current, a secondvoltage at a second terminal of the second reference sensor; providing afifth current to a first terminal of a second active sensor; driving, atthe second voltage, a second terminal of the second active sensor;exposing the second active sensor to the sensor image; and measuring asixth current from the first terminal of the second active sensor to theinput of a readout element.
 30. The method of claim 29, comprising:electrically uncoupling, from the reference sensor, a first currentsource providing the first current; coupling, to the second referencesensor, the first current source providing the fourth current;electrically uncoupling, from the active sensor, a second current sourceproviding the second current; and coupling, to the second active sensor,the second current source providing the fifth current.
 31. The method ofclaim 24, further comprising: determining an offset generated by theinput of the readout element; and canceling the offset prior tomeasuring the current to the input of the readout element.
 32. Themethod of claim 24, wherein a voltage at an output of the readoutelement is proportional to an impedance difference between the referencesensor and the active sensor.
 33. The method of claim 24, wherein thevoltage is driven by an op amp and the first terminal of the referencesensor is electrically coupled to a negative input of the op amp. 34.The method of claim 24, further comprising feeding back from the secondterminal of the reference sensor to the first terminal of the referencesensor using a feedback element.
 35. The method of claim 24, furthercomprising: providing a seventh current to a first terminal of a thirdreference sensor, the seventh current reflective of self-heatinggenerated by the third reference sensor; and adjusting a value of thesecond current in accordance with the seventh current.
 36. The method ofclaim 24, further comprising sampling a voltage generated by the currentto the input of a readout element.
 37. The method of claim 24, whereinthe first current and the second current are equal in magnitude and in asame direction relative to the respective first terminals of thereference sensor and active sensor.
 38. The method of claim 24, furthercomprising converting the third current to a readout voltage of thereadout element.
 39. The method of claim 24, wherein the first currentand the second current are each provided by current sources selectedfrom the group of an athermal voltage source and resistor, ahigh-impedance athermal transistor current source, and a Wilson currentmirror.
 40. The method of claim 24, wherein driving the second terminalof the active sensor at the voltage further comprises driving, from anoutput of a voltage driver, the second terminal of the reference sensorand the second terminal of the active sensor.
 41. The method of claim24, further comprising: causing a voltage drop across the referencesensor from the first current; generating the voltage using an amplifieroutputting to the second terminal of the reference sensor; andelectrically coupling the first terminal of the reference sensor to anegative terminal of the amplifier.
 42. The method of claim 24, whereinthe reference sensor is a reference bolometer pixel and the activesensor is an active bolometer pixel.
 43. The method of claim 24, whereinexposing the active sensor to the sensor image further comprisesexposing the active sensor to LWIR radiation.
 44. The method of claim24, wherein the readout element comprises a Sigma-Delta ADC.
 45. Themethod of claim 44, wherein a first stage of the Sigma-Delta ADCcomprises a CTIA.
 46. The method of claim 24, further comprising:exposing the reference sensor to an ambient condition common to thereference sensor and active sensor; and shielding the reference sensorfrom the sensor image.
 47. The method of claim 24, wherein driving thesecond terminal of the active sensor at the voltage further comprisesbuffering between the second terminal of the active sensor and a voltagesource providing the voltage.
 48. The method of claim 24, furthercomprising: buffering the first current; and buffering the second thecurrent.
 49. A method of manufacturing a sensor readout circuit,comprising: providing a readout element comprising an input; providing afirst current source; providing a second current source; providing avoltage driver comprising an output; providing a reference sensorcomprising a first terminal and a second terminal; electrically couplingthe first terminal of the reference sensor to the first current source;electrically coupling the second terminal of the reference sensor to theoutput of the voltage driver; providing an active sensor comprising afirst terminal and a second terminal, the active sensor configured forexposure to a sensor image; electrically coupling the first terminal ofthe active sensor to the second current source and the input of thereadout element; and electrically coupling the second terminal of theactive sensor to the output of the voltage driver.
 50. The method ofmanufacturing of claim 49, wherein the first current and the secondcurrent sources are constant current sources.
 51. The method ofmanufacturing of claim 49, wherein the voltage driver is configured togenerate a bias voltage for the active sensor.
 52. The method ofmanufacturing of claim 49, wherein the active sensor is furtherconfigured to change a current from the first terminal of the activesensor to the input of the readout element when the active sensor isexposed to the sensor image.
 53. The method of manufacturing of claim49, wherein the active sensor is further configured to change animpedance of the active sensor when the active sensor is exposed to thesensor image.
 54. The method of manufacturing of claim 49, wherein thereference sensor is a reference bolometer pixel and the active sensor isan active bolometer pixel.
 55. The method of manufacturing of claim 49,further comprising: providing a second reference sensor comprising afirst terminal and a second terminal; electrically coupling the firstterminal of the second reference sensor to the first current source;electrically coupling the second terminal of the second reference sensorto the voltage driver; providing a second active sensor comprising afirst terminal and a second terminal, the second active sensorconfigured for exposure to the sensor image; electrically coupling thefirst terminal of the active sensor to the second current source;electrically coupling the second terminal of the active sensor to theoutput of the voltage driver, and wherein the second active sensor isconfigured to change a current from the first terminal of the activesensor to the input of the readout element; and providing a first switchconfigured to selectively electrically couple the reference sensor tothe first current source; providing a second switch configured toselectively electrically couple the active sensor to the second currentsource; providing a third switch configured to selectively electricallycouple the second reference sensor to the first current source; andproviding a fourth switch configured to selectively electrically couplethe second active sensor to the second current source.
 56. The method ofmanufacturing of claim 49, further comprising providing a CDS circuitconfigured to remove an offset.
 57. The method of manufacturing of claim49, wherein the readout element is configured to generate a voltageproportional to an impedance difference between the reference sensor andthe active sensor.
 58. The method of manufacturing of claim 49, furthercomprising: providing an op amp; and electrically coupling an output ofan op amp to the second terminal of the reference sensor.
 59. The methodof manufacturing of claim 49, further comprising: providing a feedbackelement; and electrically coupling the feedback element to the first andsecond terminals of the reference sensor.
 60. The method ofmanufacturing of claim 49, further comprising: providing a thirdreference sensor comprising a first terminal and a second terminal;electrically coupling the second terminal of the third reference sensorto the output of the voltage driver; providing a third current sourceconfigured to output a seventh current reflective of self-heatinggenerated by the third reference sensor, wherein a value of the secondcurrent adjusts in accordance with the seventh current; and electricallycoupling the third current source to the first terminal of the thirdreference sensor.
 61. The method of manufacturing of claim 49, furthercomprising providing an ADC configured to sample the change of thecurrent from the first terminal to the input of the readout element. 62.The method of manufacturing of claim 49, wherein the first currentsource and the second current source are configured to output an equalmagnitude of current in a same direction relative to the respectivefirst terminals.
 63. The method of manufacturing of claim 49, whereinthe readout element comprises a CTIA.
 64. The method of manufacturing ofclaim 49, wherein the first current source and the second current sourceare selected from the group of an athermal voltage source and resistor,a high-impedance athermal transistor current source, and a Wilsoncurrent mirror.
 65. The method of manufacturing of claim 49, furthercomprising: providing an amplifier outputting to the second terminal ofthe reference sensor; and electrically coupling the first terminal ofthe reference sensor to a negative input of the amplifier, wherein thefirst current source is configured to generate a voltage drop across thenegative input and the output.
 66. The method of manufacturing of claim49, wherein the reference sensor is a reference bolometer pixel and theactive sensor is a bolometer pixel configured to detect LWIR radiation.67. The method of manufacturing of claim 49, wherein the readout elementcomprises a Sigma-Delta ADC.
 68. The method of manufacturing of claim67, wherein a first stage of the Sigma-Delta ADC comprises a CTIA. 69.The method of manufacturing of claim 49, wherein the reference sensor isshielded from a sensor image.
 70. The method of manufacturing of claim49, further comprising: providing a voltage follower; and electricallycoupling the voltage follower between the output of the voltage driverand the second terminal of the active sensor.
 71. The method ofmanufacturing of claim 49, further comprising: providing two or morecurrent buffers including a first current buffer and a second currentbuffer; electrically coupling the first current buffer between the firstcurrent source and the reference sensor; and electrically coupling thesecond current buffer between the second current source and the activesensor.
 72. A sensor circuit, comprising: a plurality of active sensorsexposed to a sensor image and sharing a bias voltage node; a calibrationreadout element; and a calibration sensor shielded from the sensor imageand comprising a first terminal electrically coupled to the bias voltagenode and a second terminal electrically coupled to the calibrationreadout element.
 73. The sensor circuit of claim 72, wherein animpedance of the calibration sensor is the same as an impedance of anactive sensor of the plurality of active sensors, and wherein anelectrical carrier count of the calibration sensor is greater than anelectrical carrier count of the active sensor.
 74. The sensor circuit ofclaim 72, further comprising: a readout element corresponding to anactive sensor of the plurality of active sensors and configured tomeasure a readout voltage of the active sensor, wherein: the calibrationreadout element is configured to measure a readout voltage of thecalibration sensor, and the sensor circuit is electrically coupled to: aprocessor; and a memory including instructions, which when executed bythe processor, cause the processor to perform a method comprising:receiving the readout voltage of the active sensor; receiving thereadout voltage of the calibration sensor; and computing a differencebetween (1) the readout voltage of the active sensor and (2) the readoutvoltage of the calibration sensor weighted by a ratio between animpedance of the calibration sensor and an impedance of the activesensor.
 75. The sensor circuit of claim 74, wherein the ratio is one.76. The sensor circuit of claim 74, wherein the ratio is temperatureindependent.
 77. The sensor circuit of claim 72, further comprising: areadout element corresponding to an active sensor of the plurality ofactive sensors and configured to measure a readout voltage of the activesensor, wherein the sensor circuit is electrically coupled to: aprocessor; and a memory including instructions, which when executed bythe processor, cause the one or more processors to perform a methodcomprising: receiving a first readout voltage corresponding to a closedshutter; receiving a second readout voltage corresponding to an openedshutter; and computing a difference proportional to an impedancedifference of the active sensor caused by the sensor image between (1)the first readout voltage and (2) the second readout voltage.
 78. Thesensor circuit of claim 74-77, wherein the plurality of readout elementscomprises a plurality of ADCs.
 79. The sensor circuit of claim 72,wherein the calibration sensor and the plurality of active sensors aremade from materials having a same thermal coefficient of resistance(TCR).
 80. The sensor circuit of claim 72, wherein the plurality ofactive sensors includes a plurality of columns of active sensors, thecircuit further comprising: a plurality of current sources, wherein acurrent source of the plurality of current sources is electricallycoupled to the second terminal of the calibration sensor and thecalibration readout element; and a plurality of readout elements,wherein each of the plurality of columns of active sensors iselectrically coupled to: a corresponding current source of the pluralityof current sources at a corresponding readout node, and a correspondingreadout element of the plurality of readout elements at thecorresponding readout node.
 81. The sensor circuit of claim 72, whereinthe calibration readout element comprises an analog-to-digital converter(ADC).
 82. The sensor circuit of claim 72, wherein: the plurality ofactive sensors and the calibration sensor are bolometers, and the sensorimage is a thermal image.
 83. A sensor circuit, comprising: acalibration current source providing a calibration current; an activesensor; a readout element; a first switch configured to selectivelyelectrically couple the active sensor to the readout element; and asecond switch configured to selectively electrically couple thecalibration current source to the readout element.
 84. The sensorcircuit of claim 83, wherein: the second switch is configured toelectrically uncouple the calibration current source from the firstreadout element when the first switch electrically couples the activesensor to the readout element, and the first switch is configured toelectrically uncouple the active sensor from the first readout elementwhen the second switch electrically couples the calibration current tothe readout element, and the sensor circuit is electrically coupled to:a processor; and a memory including instructions, which when executed bythe processor, cause the processor to perform a method comprising:receiving a first readout voltage of the active sensor; receiving asecond readout voltage caused by the calibration current; and computingan output proportional to a readout current of the active sensor basedon (1) the first readout voltage and (2) the second readout voltage. 85.The sensor circuit of claim 84, further comprising: a plurality ofactive sensors comprising the active sensor; and a plurality of readoutelements comprising the first and second readout elements, each of theplurality of readout elements electrically coupled to a respectiveactive sensor of the plurality of active sensors, wherein the methodfurther comprises: receiving, from a readout element of the plurality ofreadout elements, a first readout voltage of the respective activesensor; receiving a second readout voltage caused by the calibrationcurrent on a respective readout element; and computing a respectiveoutput proportional to a readout current of the respective active sensorbased on (1) the readout voltage of the respective sensor and (2) thesecond readout voltage caused by the calibration current on therespective readout element.
 86. The sensor circuit of claim 84, furthercomprising a second active sensor belonging to a same column as thefirst active sensor, wherein the method further comprises, aftercomputing the first output: receiving a third readout voltage of thesecond active sensor; and computing a second output proportional to areadout current of the fourth active sensor based on (1) the thirdreadout voltage and (2) the second readout voltage caused by thecalibration current.
 87. The sensor circuit of claim 86, wherein a timebetween successive receipts of the second readout voltage on the samecolumn caused by the calibration current is a calibration period. 88.The sensor circuit of claim 87, wherein the calibration period is onesecond.
 89. The sensor circuit of claim 87, wherein the calibrationperiod is based on a drift of the readout element.
 90. The sensorcircuit of claim 87, wherein different rows are readout during thesuccessive receipts of the second readout voltage.
 91. The sensorcircuit of claim 83, further comprising: a second calibration currentsource; a third switch configured to selectively electrically couple thefirst calibration current source to the readout element; and a fourthswitch configured to selectively electrically couple the secondcalibration current source to the readout element, and wherein: when thethird switch electrically uncouples the readout element from the firstcalibration current source: the fourth switch is configured toelectrically couple the readout element to the second calibrationcurrent source, and the method further comprises receiving a thirdreadout voltage caused by the second calibration current; and the outputis further based on the third readout voltage caused by the secondcalibration current.
 92. The sensor circuit of claim 83, wherein thereadout element comprises an ADC.
 93. The sensor circuit of claim 83,wherein the sensor circuit is electrically coupled to: a processor; anda memory including instructions, which when executed by the processor,cause the one or more processors to perform a method comprising:receiving a first readout voltage corresponding to a closed shutter;receiving a second readout voltage corresponding to an opened shutter;and computing a difference proportional to an impedance difference ofthe first active sensor caused by a sensor image between (1) the firstreadout voltage and (2) the second readout voltage.
 94. The sensorcircuit of claim 83, wherein the active sensor is a bolometer exposed toa thermal scene.
 95. The sensor circuit of claim 83, wherein the activesensor is exposed to a sensor image and shares a bias voltage node witha plurality of active sensors, further comprising: a second readoutelement; and a calibration sensor shielded from the sensor image andcomprising a first terminal electrically coupled to the bias voltagenode and a second terminal electrically coupled to the second readoutelement.
 96. A method of calculating a calibrated voltage in a sensorcircuit, the method comprising: electrically coupling a first terminalof a calibration sensor to a bias voltage node shared by a plurality ofactive sensors; electrically coupling a second terminal of thecalibration sensor to a calibration readout element; exposing theplurality of active sensors to a sensor image; shielding the calibrationsensor from the sensor image; measuring, with a readout element, areadout voltage of an active sensor of the plurality of active sensors;measuring, with the calibration readout element, a readout voltage ofthe calibration sensor; and computing the calibrated voltage as adifference between (1) the readout voltage of the active sensor and (2)the readout voltage of the calibration sensor weighted by a ratiobetween an impedance of the calibration sensor and an impedance of theactive sensor.
 97. The method of claim 96, wherein the impedance of thecalibration sensor is the same as the impedance of the active sensor,and wherein an electrical carrier count of the calibration sensor isgreater than an electrical carrier count of the active sensor.
 98. Themethod of claim 96, wherein the ratio is one.
 99. The method of claim96, wherein the ratio is temperature independent.
 100. The method ofclaim 96, wherein the calibration sensor and the active sensor are madefrom materials having a same TCR.
 101. The method of claim 96, furthercomprising: electrically coupling a current source of a plurality ofcurrent sources to the second terminal of the calibration sensor and tothe calibration readout element; electrically coupling a column of aplurality of columns of active sensors to the readout element, thecolumn of active sensors including the active sensor; and electricallycoupling a second current source of the plurality of current sources tothe readout element.
 102. The method of claim 96, further comprising:closing a shutter; measuring, with the readout element, a first readoutvoltage corresponding to the closed shutter; and measuring, with thecalibration readout element, a second readout voltage corresponding tothe closed shutter; and after computing the calibrated voltage,computing a second difference between (1) the calibrated voltage and adifference between (2a) the first readout voltage and (2b) the secondreadout voltage weighted by the ratio, wherein the second difference isa shutter calibrated voltage.
 103. The method of claim 96, wherein thecalibration readout element comprises an ADC.
 104. The method of claim96, wherein the readout element comprises an ADC.
 105. The method ofclaim 96, wherein: the plurality of active sensors and the calibrationsensor are bolometers, and the sensor image is a thermal image.
 106. Amethod of calculating an output in a sensor circuit, the methodcomprising: electrically coupling a readout element to an active sensor;measuring, with the readout element, a first readout voltage of theactive sensor; electrically uncoupling the readout element from theactive sensor; electrically coupling a calibration current to thereadout element; measuring, with the readout element, a second readoutvoltage caused by the calibration current; and computing the outputbased on (1) the first readout voltage and (2) the second readoutvoltage, the output proportional to a readout current of the activesensor.
 107. The method of claim 106, further comprising: electricallycoupling a respective active sensor of a plurality of active sensors toa readout element of a plurality of readout elements; measuring, withthe respective readout element, a first readout voltage of therespective active sensor; electrically uncoupling the respective readoutelement from the respective active sensor; electrically coupling thecalibration current to the respective readout element; measuring, withthe respective readout element, a second readout voltage caused by thecalibration current on the respective readout element; and computing anoutput proportional to a readout current of the respective active sensorbased on (1) the first readout voltage of the respective active sensorand (2) the second readout voltage caused by the calibration current.108. The method of claim 106, further comprising, after computing thefirst output: electrically uncoupling the calibration current sourcefrom the readout element; electrically coupling the readout element to asecond active sensor, the second active sensor belonging to a samecolumn as the first active sensor; measuring, with the readout element,a third readout voltage of the second active sensor; and computing asecond output proportional to a readout current of the second activesensor based on (1) the third readout voltage and (2) the second readoutvoltage caused by the calibration current.
 109. The method of claim 108,wherein a time between successive measurements of the second readoutvoltage on the same column caused by the calibration current is acalibration period.
 110. The method of claim 109, wherein thecalibration period is one second.
 111. The method of claim 110, whereinthe calibration period is based on a drift of the readout element. 112.The method of claim 111, wherein different rows are readout during thesuccessive measurements of the second readout voltage.
 113. The methodof claim 106, further comprising: electrically uncoupling the readoutelement from the first calibration current source; electrically couplingthe readout element to a second calibration current source; andmeasuring, with the readout element, a third readout voltage caused bythe second calibration current on the readout element, wherein theoutput is further based on the third readout voltage caused by thesecond calibration current.
 114. The method of claim 106, wherein thereadout element comprises an ADC.
 115. The method of claim 106, furthercomprising: closing a shutter; computing the output corresponding to aclosed shutter; and computing a difference proportional to an impedancedifference of the active sensor caused by a sensor image between (1) theoutput corresponding to an opened shutter and (2) the outputcorresponding to the closed shutter.
 116. The method of claim 106,wherein the active sensor is a bolometer exposed to a thermal scene.117. The method of claim 106, further comprising: electricallyuncoupling the readout element from the calibration current source;electrically coupling a second readout element to the calibrationcurrent source; measuring, with the second readout element, a thirdreadout voltage caused by the calibration current; electricallyuncoupling the second readout element from the calibration currentsource; electrically coupling a first terminal of a calibration sensorto a bias voltage node shared by a plurality of active sensors and theactive sensor; electrically coupling a second terminal of thecalibration sensor to the second readout element; exposing the pluralityof active sensors and the active sensor to a sensor image; shielding thecalibration sensor from the sensor image; measuring, with the secondreadout element, a fourth readout voltage of the calibration sensor;computing a second output based on the third readout voltage and thefourth readout voltage; and computing a difference between (1) the firstoutput and (2) the second output weighted by a ratio between animpedance of the calibration sensor and an impedance of the activesensor.
 118. A method of manufacturing one of the sensor circuits inclaims 72-95.
 119. The circuit of claim 1, further comprising a fifthswitch configured to selectively electrically couple the active sensorto the voltage driver.
 120. A sensor circuit, comprising: a plurality ofsensor pixels, each configured to store a charge; a Sigma-Delta ADCconfigured to receive the charge of each sensor; and a plurality ofswitches configured to sequentially couple each of the plurality ofsensor pixels to the Sigma-Delta ADC, each switch corresponding to arespective one of the plurality of sensor pixels.
 121. The sensorcircuit of claim 120, wherein the sensor circuit does not include a CTIAelectrically positioned between the plurality of sensor pixels and theSigma-Delta ADC.
 122. The sensor circuit of claim 120, furthercomprising a variable resistor electrically positioned between theplurality of sensors and the Sigma-Delta ADC, wherein the plurality ofswitches are configured to sequentially couple each of the plurality ofsensor pixels to the variable resistor.
 123. The sensor circuit of claim122, wherein: the variable resistor has a linearly decreasing resistanceduring a discharge time window; the variable resistor is at a lowestresistance at an end of the discharge time window; and the variableresistor has a resistance higher than the lowest resistance between thebeginning and the end of the discharge time window.
 124. The circuit ofclaim 123, wherein: the variable resistor is a MOS transistor; and theinitial resistance, the linearly decreasing resistance, and the lowestresistance of the MOS transistor are controlled with a control voltageelectrically coupled to the MOS transistor.
 125. The circuit of claim123, wherein the discharge time window is between 10 microseconds and 1millisecond.
 126. The circuit of claim 123, wherein: during the firstdischarge time window, a first switch electrically couples a firstsensor pixel and the Sigma-Delta ADC; during a second discharge timewindow, a second switch electrically couples a second sensor pixel andthe Sigma-Delta ADC; and the first and second discharge time windowscorrespond to readout times of the first and second sensor pixels. 127.The circuit of claim 123, wherein during the discharge time window, aconstant current of the variable resistor is an initial voltage of thevariable resistor divided by the initial resistance.
 128. The circuit ofclaim 122 wherein a switch electrically couples a respective sensorpixel and the variable resistor during a respective discharge timewindow, the discharge time window equal to a capacitance of the sensorpixel multiplied by an initial resistance of the variable resistor. 129.The circuit of claim 122, wherein: the variable resistor includes aweighted bank of resistors; the weighted bank of resistors include aplurality of resistors selectively electrically coupled in parallel orin series; and resistances of combinations of the selective electricallycoupled resistors include an initial resistance at the beginning of adischarge time window, a linearly decreasing resistance, and a lowestresistance.
 130. The circuit of claim 120, wherein a sensor pixelincludes an x-ray sensor photodiode and the charge is indicative of thex-ray sensor photodiode's exposure to x-ray.
 131. The circuit of claim120, wherein a sensor pixel includes a storage capacitor storing thecharge and the sensor pixel's exposure to x-ray generates the chargestored in the storage capacitor.
 132. The circuit of claim 120, furthercomprising a second plurality of sensor pixels and a second Sigma-DeltaADC, wherein the second plurality of sensor pixels are configured tosequentially couple to the second Sigma-Delta ADC and the first andsecond pluralities of sensor pixels belong to a same column.
 133. Thecircuit of claim 132, wherein numbers of the first and second pluralityof sensor pixels are equal.
 134. The circuit of claim 132, wherein, at afirst row time, a first sensor pixel of the first plurality of sensorpixels and a second sensor pixel of the second plurality of sensorpixels are simultaneously readout.
 135. The circuit of claim 120,wherein an input current to the Sigma-Delta ADC is constant.
 136. Thecircuit of claim 120, further comprising a digital filter configured toreceive a signal from the Sigma-Delta ADC.
 137. A method of readout of asensor circuit, the sensor circuit comprising a plurality of sensorpixels, a Sigma-Delta ADC, and a plurality of switches, each switchcorresponding to a respective one of the plurality of sensor pixels, themethod comprising: storing respective charges in each of the pluralityof sensor pixels; sequentially electrically coupling, using theplurality of switches, each of the plurality of sensor pixels to theSigma-Delta ADC; and sequentially receiving, at the Sigma-Delta ADC, therespective charge of each sensor pixel.
 138. The method of claim 137,wherein the sensor circuit does not include a CTIA electricallypositioned between the plurality of sensor pixels and the Sigma-DeltaADC and the respective charge of each sensor pixel is not received bythe CTIA.
 139. The method of claim 137, wherein the sensor circuitfurther comprises a variable resistor electrically positioned betweenthe plurality of sensor pixels and the Sigma-Delta ADC and the methodfurther comprises: sequentially electrically coupling, using theplurality of switches, each of the plurality of sensor pixels to theSigma-Delta ADC further comprises sequentially electrically coupling,using the plurality of switches, the each of the plurality of sensorpixels to the variable resistor.
 140. The method of claim 139, furthercomprising linearly decreasing a resistance of the variable resistorduring a discharge time window, wherein: the variable resistor is at alowest resistance at an end of the discharge time window; and thevariable resistor has a resistance higher than the lowest resistancebetween the beginning and the end of the discharge time window.
 141. Themethod of claim 140, wherein the variable resistor is a MOS transistorelectrically coupled to a control voltage and linearly decreasing theresistance of the variable resistor further comprises driving the MOStransistor with the control voltage to generate the initial resistance,the linearly decreasing resistance, and the lowest resistance.
 142. Themethod of claim 140, wherein the discharge time window is between 10microseconds and 1 millisecond.
 143. The method of claim 140, whereinsequentially electrically coupling, using the plurality of switches,each of the plurality of sensor pixels to the Sigma-Delta ADC furthercomprises: during the first discharge time window, electrically couplinga first switch to a first sensor pixel and the Sigma-Delta ADC; during asecond discharge time window, electrically coupling a second switch to asecond sensor pixel and the Sigma-Delta ADC, wherein the first andsecond discharge time windows correspond to readout times of the firstand second sensor pixels.
 144. The method of claim 140, wherein duringthe discharge time window, a constant current of the variable resistoris an initial voltage of the variable resistor divided by the initialresistance.
 145. The method of claim 139 wherein sequentiallyelectrically coupling, using the plurality of switches, each of theplurality of sensor pixels to the Sigma-Delta ADC further compriseselectrically coupling a switch to a respective sensor pixel and thevariable resistor during a respective discharge time window; and thedischarge time window is equal to a capacitance of the sensor pixelmultiplied by an initial resistance of the variable resistor.
 146. Themethod of claim 139, wherein: the variable resistor includes a weightedbank of resistors; the weighted bank of resistors include a plurality ofresistors selectively electrically coupled in parallel or in series; andthe method further comprises linearly decreasing resistances ofcombinations of the plurality of resistors, from an initial resistanceat a beginning of a discharge time window to a lowest resistance at anend of the discharge time window, by selective electrically coupling theresistors.
 147. The method of claim 137, wherein a sensor pixel includesan x-ray sensor photodiode and the charge is indicative of the x-raysensor photodiode's exposure to x-ray.
 148. The method of claim 137,wherein storing respective charges in each of the plurality of sensorpixels further comprises: exposing the each of the plurality of sensorpixels to x-ray and generating the respective charge; and storing therespective charges in a storage capacitor of the each of the pluralityof sensor pixels.
 149. The method of claim 137, wherein the sensorcircuit further comprises a second plurality of sensor pixels belongingto a same column as the first plurality of sensor pixels, a secondplurality of switches, and a second Sigma-Delta ADC, the method furthercomprises: sequentially electrically coupling, using the secondplurality of switches, each of the plurality of sensor pixels to thesecond Sigma-Delta ADC; and sequentially receiving, at the secondSigma-Delta ADC, the respective charge of each sensor pixel of thesecond plurality of sensor pixels.
 150. The method of claim 149, whereinnumbers of the first and second plurality of sensor pixels are equal.151. The method of claim 149, wherein, at a first row time: the firstSigma-Delta ADC receives a first respective charge of a first sensorpixel of the first plurality of sensor pixels; and the secondSigma-Delta ADC receives a second respective charge of a second sensorpixel of the second plurality of sensor pixels.
 152. The method of claim137, wherein the Sigma-Delta ADC receives a constant current.
 153. Themethod of claim 137, further comprising digitally filtering a signalfrom the Sigma-Delta ADC.
 154. A method of manufacturing one of thesensor circuits in claims 120-136.